Evidence for Large Superhumps in TX Col and V4742 Sgr

Since the discovery of the largest positive superhump period in TV Col (6.4 h), we have started a program to search for superhumps in cataclysmic variables (CVs) with large orbital periods. In this work, we summarize preliminary results of our observations of TX Col and V4742 Sgr. TX Col is an intermediate polar with a 5.7-h orbital period. V4742 Sgr is a recent (2002) nova with no known periods. CCD unfiltered continuous photometry of these two objects was carried out during 56 nights (350 hours) in 2002–2003. The time series analysis reveals the presence of several periods in both power spectra. In TX Col, in addition to the orbital period of 5.7 h, we found peaks at 7.1 h and 5.0 h. These are interpreted as positive and negative superhumps correspondingly, although the effects of the quasi-periodic oscillations at ∼2 h (which may cause spurious signals) were not taken into consideration. In the light curve of V4742 Sgr two long periods are detected − 6.1 and 5.4 h as well as a short-term period at 1.6 h. This result suggests that V4742 Sgr is an intermediate polar candidate and a permanent superhump system with a large orbital period (5.4 h) and a superhump period excess of 13%. If these results are confirmed, TX Col and V4742 Sgr join TV Col to form a group of intermediate polars with extremely large superhump periods. There seems to be now growing evidence that superhumps can occur in intermediate polars with long orbital periods, which is very likely inconsistent with the theoretical prediction that superhumps can only occur in systems with mass ratios below 0.33. Alternatively, if the mass ratio in these systems is nevertheless below the theoretical limit, they should harbour undermassive secondaries and very massive white dwarfs, near the Chandrasekhar limit, which would make them excellent candidates for progenitors of supernovae type Ia.