diff --git a/invisible_cities/core/fit_functions.py b/invisible_cities/core/fit_functions.py
index 9032adfaa..debdab8b9 100644
--- a/invisible_cities/core/fit_functions.py
+++ b/invisible_cities/core/fit_functions.py
@@ -157,8 +157,10 @@ def fit(func, x, y, seed=(), fit_range=None, **kwargs):
 
     Returns
     -------
-    fitted_fun : extended function (contains values and errors)
-        Fitted function.
+    fitted_fun : FitFunction
+        Extended function containing fit parameters (fitf, vals, errors, chi2, pval,
+        cov) and full_output from curve_fit and leastsq (infodict, mesg and ier).
+
 
     Examples
     --------
@@ -183,7 +185,7 @@ def fit(func, x, y, seed=(), fit_range=None, **kwargs):
 
     kwargs['absolute_sigma'] = "sigma" in kwargs
 
-    vals, cov = scipy.optimize.curve_fit(func, x, y, seed, **kwargs)
+    vals, cov, infodict, mesg, ier = scipy.optimize.curve_fit(func, x, y, seed, full_output=True, **kwargs)
 
     fitf       = lambda x: func(x, *vals)
     fitx       = fitf(x)
@@ -191,8 +193,7 @@ def fit(func, x, y, seed=(), fit_range=None, **kwargs):
     ndof       = len(y) - len(vals)
     chi2, pval = get_chi2_and_pvalue(y, fitx, ndof, sigma_r)
 
-
-    return FitFunction(fitf, vals, errors, chi2, pval, cov)
+    return FitFunction(fitf, vals, errors, chi2, pval, cov, infodict, mesg, ier)
 
 
 def profileX(xdata, ydata, nbins=100,
diff --git a/invisible_cities/evm/ic_containers.py b/invisible_cities/evm/ic_containers.py
index 39eb0ff89..d49e249f2 100644
--- a/invisible_cities/evm/ic_containers.py
+++ b/invisible_cities/evm/ic_containers.py
@@ -25,7 +25,7 @@ def _add_namedtuple_in_this_module(name, attribute_names):
         ('CalibVectors'   , 'channel_id coeff_blr coeff_c adc_to_pes adc_to_pes_sipm pmt_active'),
         ('S12Params'      , 'time stride length rebin_stride'),
         ('ZsWf'           , 'indices energies'),
-        ('FitFunction'    , 'fn values errors chi2 pvalue cov'),
+        ('FitFunction'    , 'fn values errors chi2 pvalue cov infodict mesg ier'),
         ('TriggerParams'  , 'trigger_channels min_number_channels charge height width'),
         ('SensorParams'   , 'spectra peak_range min_bin_peak max_bin_peak half_peak_width p1pe_seed lim_ped'),
         ('PedestalParams' , 'gain gain_min gain_max sigma sigma_min sigma_max')):
diff --git a/invisible_cities/reco/krmap_functions.py b/invisible_cities/reco/krmap_functions.py
new file mode 100644
index 000000000..a967041fd
--- /dev/null
+++ b/invisible_cities/reco/krmap_functions.py
@@ -0,0 +1,151 @@
+import numpy  as np
+import pandas as pd
+
+from .. types.symbols       import KrFitFunction
+from .. evm.ic_containers   import FitFunction
+from .. core.fit_functions  import polynom, expo
+
+
+def lin_seed(x : np.array, y : np.array):
+    '''
+    Estimate the seed for a linear fit of the form y = a + bx.
+
+    Parameters
+    ----------
+    x : np.array
+        Independent variable.
+    y : np.array
+        Dependent variable.
+
+    Returns
+    -------
+    seed : tuple
+        Seed parameters (intercept, slope) for the linear fit.
+    '''
+    x0, x1 = x.min(), x.max()
+    y0, y1 = y.min(), y.max()
+
+    if x1 == x0: # If same x value, set slope to 0 and use the mean value of y as interceipt
+        b = 0
+        a = y.mean()
+    else:
+        b = (y1 - y0) / (x1 - x0)
+        a = y0 - b * x0
+
+    return a, b
+
+
+def expo_seed(x : np.array, y : np.array):
+    '''
+    Estimate the seed for an exponential fit of the form y = y0*exp(-x/lt).
+
+    Parameters
+    ----------
+    x : np.array
+        Independent variable.
+    y : np.array
+        Dependent variable.
+
+    Returns
+    -------
+    seed : tuple
+        Seed parameters (y0, lt) for the exponential fit.
+    '''
+    x, y = zip(*sorted(zip(x, y)))
+    y0   = y[0]
+
+    if y0 <= 0 or y[-1] <= 0:
+        raise ValueError("y data must be > 0")
+
+    lt = -x[-1] / np.log(y[-1] / y0)
+
+    return y0, lt
+
+
+def select_fit_variables(fittype : KrFitFunction, dst : pd.DataFrame):
+    '''
+    Select the data for fitting based on the specified fit type.
+
+    Parameters
+    ----------
+    fittype : KrFitFunction
+        The type of fit function to prepare data for (e.g., linear, exponential, log-linear).
+    dst : pd.DataFrame
+        The DataFrame containing the data to be prepared for fitting.
+
+    Returns
+    -------
+    x_data : pd.Series
+        The independent variable data prepared for fitting.
+    y_data : pd.Series
+        The dependent variable data prepared for fitting.
+    '''
+    if   fittype is KrFitFunction.linear : return dst.DT,         dst.S2e
+    elif fittype is KrFitFunction.expo   : return dst.DT,         dst.S2e
+    elif fittype is KrFitFunction.log_lin: return dst.DT, -np.log(dst.S2e)
+
+
+def get_function_and_seed_lt(fittype : KrFitFunction):
+    '''
+    Retrieve the fitting function and seed function based on the
+    specified fittype.
+
+    Parameters
+    ----------
+    fittype : KrFitFunction
+        The type of fit function to retrieve (e.g., linear, exponential, log-linear).
+
+    Returns
+    -------
+    fit_function  : function
+        The fitting function corresponding to the specified fit type.
+    seed_function : function
+        The seed function corresponding to the specified fit type.
+    '''
+    linear_function  = lambda x, y0, slope: polynom(x, y0, slope)
+    expo_function    = lambda x, e0, lt:    expo   (x, e0, -lt)
+
+    if   fittype is KrFitFunction.linear:  return linear_function,  lin_seed
+    elif fittype is KrFitFunction.log_lin: return linear_function,  lin_seed
+    elif fittype is KrFitFunction.expo:    return   expo_function, expo_seed
+
+
+def transform_parameters(fit_output : FitFunction):
+    '''
+    Transform the parameters obtained from the fitting output into EO and LT.
+    When using log_lin fit, we need to convert the intermediate variables into
+    the actual physical magnitudes involved in the process.
+
+    Parameters
+    ----------
+    fit_output : FitFunction
+        Output from IC's fit containing the parameter values, errors, and
+        covariance matrix.
+
+    Returns
+    -------
+    par : list
+        Transformed parameter values.
+    err : list
+        Transformed parameter errors.
+    cov : float
+        Transformed covariance value.
+    '''
+    par = fit_output.values
+    err = fit_output.errors
+    cov = fit_output.cov[0, 1]
+
+    a, b     = par
+    u_a, u_b = err
+
+    E0   = np.exp(-a)
+    s_E0 = np.abs(E0 * u_a)
+    lt   = 1 / b
+    s_lt = np.abs(lt**2 * u_b)
+    cov  = E0 * lt**2 * cov # Not sure about this
+
+    par  = [  E0,   lt]
+    err  = [s_E0, s_lt]
+
+    return par, err, cov
+
diff --git a/invisible_cities/reco/krmap_functions_test.py b/invisible_cities/reco/krmap_functions_test.py
new file mode 100644
index 000000000..fcb5cbd34
--- /dev/null
+++ b/invisible_cities/reco/krmap_functions_test.py
@@ -0,0 +1,106 @@
+import pytest
+
+import numpy          as np
+import numpy.testing  as npt
+import pandas         as pd
+import scipy.optimize as so
+
+from hypothesis             import given, settings
+from hypothesis.strategies  import floats, integers
+
+from .. reco                import krmap_functions as krf
+from ..types.symbols        import KrFitFunction
+from .. evm.ic_containers   import FitFunction
+from .. core.fit_functions  import expo
+
+
+@given(floats(min_value = 0,  max_value = 10),
+       floats(min_value = 10, max_value = 20),
+       floats(min_value = 1,  max_value = 100),
+       floats(min_value = 0,  max_value = 10))
+def test_lin_function_output_values(x_min, x_max, a, b):
+    x = np.array([x_min, x_max])
+    y = a + b * x
+    a_test, b_test = krf.lin_seed(x, y)
+
+    assert np.isclose(a_test, a)
+    assert np.isclose(b_test, b)
+
+
+@given(floats(min_value = 1,    max_value = 10),
+       floats(min_value = 1000, max_value = 1600),
+       floats(min_value = 1e4,  max_value = 1e5),
+       floats(min_value = 1e4,  max_value = 1e5))
+def test_expo_seed_output_values(zmin, zmax, elt, e0):
+    x = np.array([zmin, zmax])
+    y = e0 * np.exp(-x / elt)
+    e0_test, elt_test = krf.expo_seed(x, y)
+
+    assert np.isclose( e0_test,  e0, rtol=0.1)
+    assert np.isclose(elt_test, elt, rtol=0.1)
+
+
+@pytest.fixture
+def sample_df():
+    data = {'DT' : [10, 20, 30, 40, 50],
+            'S2e': [50, 45, 42, 41, 41]}
+    return pd.DataFrame(data)
+
+
+def test_select_fit_variables(sample_df):
+    x_linear,  y_linear  = krf.select_fit_variables(KrFitFunction.linear,  sample_df)
+    x_expo,    y_expo    = krf.select_fit_variables(KrFitFunction.expo,    sample_df)
+    x_log_lin, y_log_lin = krf.select_fit_variables(KrFitFunction.log_lin, sample_df)
+
+    # First return the same for the 3 cases
+    assert (x_expo    == x_linear).all()
+    assert (x_log_lin == x_linear).all()
+
+    # Second return different for log_lin case
+    assert  (y_linear  == y_expo).all()
+    assert  (y_log_lin != y_expo).all()
+
+
+@settings(deadline=None)
+@given(floats  (min_value = 1,    max_value = 10),
+       floats  (min_value = 1000, max_value = 1600),
+       integers(min_value = 10,   max_value = 1e3),
+       floats  (min_value = 1e3,  max_value = 1e5),
+       floats  (min_value = 5e3,  max_value = 1e5))
+def test_get_function_and_seed_lt_with_data(x_min, x_max, steps, e0, lt):
+    x = np.linspace(x_min, x_max, steps)
+    y = expo(x, e0, lt)
+
+    fit_func_lin,         seed_func_lin = krf.get_function_and_seed_lt(KrFitFunction.linear)
+    fit_func_expo,       seed_func_expo = krf.get_function_and_seed_lt(KrFitFunction.expo)
+    fit_func_log_lin, seed_func_log_lin = krf.get_function_and_seed_lt(KrFitFunction.log_lin)
+
+    popt_lin, _     = so.curve_fit(fit_func_lin,     x, y, p0=seed_func_lin    (x, y))
+    popt_expo, _    = so.curve_fit(fit_func_expo,    x, y, p0=seed_func_expo   (x, y))
+    popt_log_lin, _ = so.curve_fit(fit_func_log_lin, x, y, p0=seed_func_log_lin(x, y))
+
+    assert     np.isclose(popt_lin[0], popt_expo[0],    rtol=1e-1) # The interceipt should be close between lin and expo
+    assert not np.isclose(popt_lin[1], popt_expo[1],    rtol=1e-1) # The "lifetime" should be different between lin and expo
+
+    assert     np.isclose(popt_lin[0], popt_log_lin[0], rtol=1e-10) # The lin and log_lin are the same (the only difference is their
+    assert     np.isclose(popt_lin[1], popt_log_lin[1], rtol=1e-10) # inputs: s2e or -log(s2e)) so both parameters should be the same
+                                                                      # for the purpose of testing this function
+
+@given(floats(min_value = 1,  max_value = 1e5),
+       floats(min_value = 10, max_value = 1e5))
+def test_transform_parameters(a, b):
+    errors     = 0.2*np.array([a, b])
+    fit_output = FitFunction(values=[a, b], errors=errors, cov=np.array([[0.04, 0.02], [0.02, 0.04]]),
+                             fn=None, chi2=None, pvalue=None, infodict=None, mesg=None, ier=None)
+
+    transformed_par, transformed_err, transformed_cov = krf.transform_parameters(fit_output)
+
+    E0_expected   = np.exp(-a)
+    s_E0_expected = np.abs(E0_expected * errors[0])
+    lt_expected   = 1 / b
+    s_lt_expected = np.abs(lt_expected**2 * errors[1])
+    cov_expected  = E0_expected * lt_expected**2 * 0.02
+
+    npt.assert_allclose(transformed_par, [  E0_expected,   lt_expected])
+    npt.assert_allclose(transformed_err, [s_E0_expected, s_lt_expected])
+    assert np.isclose(transformed_cov, cov_expected)
diff --git a/invisible_cities/types/symbols.py b/invisible_cities/types/symbols.py
index cbd0b9628..cd25b91c1 100644
--- a/invisible_cities/types/symbols.py
+++ b/invisible_cities/types/symbols.py
@@ -53,6 +53,12 @@ class InterpolationMethod(AutoNameEnumBase):
     nointerpolation = auto()
 
 
+class KrFitFunction(AutoNameEnumBase):
+    expo    = auto()
+    linear  = auto()
+    log_lin = auto()
+
+
 class MCTableType(AutoNameEnumBase):
     configuration    = auto()
     events           = auto()