  We present a study of the unidentified gamma-ray source 3EG J0542+2610. This
source is spatially superposed to the supernova remnant G180.0-1.7, but its
time variability makes unlikely a physical link. We have searched into the
EGRET location error box for compact radio sources that could be the low energy
counterpart of the gamma-ray source. Although 29 point-like radio sources were
detected and measured, none of them is strong enough as to be considered the
counterpart of a background gamma-ray emitting AGN. We suggest that the only
object within the 95 % error box capable of producing the required gamma-ray
flux is the X-ray transient A0535+26. We show that this Be/accreting pulsar can
produce variable hadronic gamma-ray emission through the mechanism originally
proposed by Cheng & Ruderman (1989), where a proton beam accelerated in a
magnetospheric electrostatic gap impacts the transient accretion disk.
