  Until now, double mode Cepheids (or beat Cepheids) were known in the Galaxy,
Magellanic Clouds and M33. Curiously, none of more than 2000 Cepheids in M31
was claimed to show two pulsation modes. We conducted a systematic search for
double mode Cepheids in the archival data and discovered four such objects. We
identify one of the stars as a first and second overtone pulsator even though
its secondary period is subject to strong aliasing. Two stars turn out to
pulsate in the fundamental mode and the first overtone. Their fundamental
periods are 9.392 d and 9.163 d. This makes them the first candidates for the
fundamental mode and the first overtone Cepheids, which double mode pulsations
are caused by the 1:2 resonance of the fundamental mode and the second
overtone.
