  Continuous CCD photometry of the classical nova DN Gem during 52 nights in
the years 1992-98 reveals a modulation with a period 0.127844 d. The
semi-amplitude is about 0.03 mag. The stability of the variation suggests that
it is the orbital period of the binary system. This interpretation makes DN Gem
the fourth nova inside the cataclysmic variable (CV) period gap, as defined by
Diaz and Bruch (1997), and it bolsters the idea that there is no period gap for
classical novae. However, the number of known nova periods is still too small
to establish this idea statistically. We eliminate several possible mechanisms
for the variation, and propose that the modulation is driven by an irradiation
effect. We find that model light curves of an irradiated secondary star, fit
the data well. The inclination angle of the system is restricted by this model
to 10 deg < i < 65 deg. We also refine a previous estimate of the distance to
the binary system, and find d=1.6+/-0.6 kpc.
