Cyclic variations with twice the accretion disk precession period in the old nova V603 Aquilae

A dense series of long and high time resolution light curves of the old nova V603 Aql, covering 22 nights (19 of which are consecutive), are analyzed in order to identify and characterize variations on the time scale of hours and days. The well known 3.5 h modulation, observed many times in the past and considered to be due to a long lasting, albeit not entirely stable superhump, is recovered at a period of 0.1453 days and an amplitude of similar to 0.(m)062. Most interesting, however, is the detection of highly significant brightness variations with an amplitude of similar to 0.(m)050 and a period of 5.85 days which is to a very high precision equal to twice the beat period between the orbital and the superhump period. The latter is generally interpreted as the precession period of an eccentric accretion disk. The origin of these long term variations remains unknown.