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 , H _ { V } , 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 H _ { V } 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 .

Conclusions :