  I use a 1-D Lagrangian code which follows both a gaseous and a dark component
to study the effects of a metagalactic UV background on the formation of dwarf
galaxies. In the case of an ionising background consistent with the existing
observational constraints no effect has been found in halos with circular
velocity above 60 km/s. The effect is very mild in halos with $40 <
V_{\mboxs{c}} < 60$ km/s, and the reduction in the total gas mass which cools
below 8000 K never surpasses 30 %. Below the circular velocity 40 km/s there is
a sharp transition towards the case in which the gas does not cool but it is
gravitationally confined by the potential well of the dark halo. Halos with a
virial temperature below the equilibrium photoionisation temperature cannot
confine the gas.
