  Context: Accurate measurements of diameters of trans-Neptunian objects are
extremely complicated to obtain. Thermal modeling can provide good results, but
accurate absolute magnitudes are needed to constrain the thermal models and
derive diameters and geometric albedos. The absolute magnitude, Hv, is defined
as the magnitude of the object reduced to unit helio- and geocentric distances
and a zero solar phase angle and is determined using phase curves. Phase
coefficients can also be obtained from phase curves. These are related to
surface properties, yet not many are known.
  Aims: Our objective is to measure accurate V band absolute magnitudes and
phase coefficients for a sample of trans-Neptunian objects, many of which have
been observed, and modeled, within the 'TNOs are cool' program, one of Herschel
Space Observatory key projects.
  Methods: We observed 56 objects using the V and R filters. These data, along
with those available in the literature, were used to obtain phase curves and
measure V band absolute magnitudes and phase coefficients by assuming a linear
trend of the phase curves and considering magnitude variability due to
rotational light-curve.
  Results: We obtained 237 new magnitudes for the 56 objects, six of them with
no reported previous measurements. Including the data from the literature we
report a total of 110 absolute magnitudes with their respective phase
coefficients. The average value of Hv is 6.39, bracketed by a minimum of 14.60
and a maximum of -1.12. In the case of the phase coefficients we report 0.10
mag per degree as the median value and a very large dispersion, ranging from
-0.88 up tp 1.35 mag per degree.
