Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables – III. A new kind of dwarf nova oscillation, and further examples of the similarities to X-ray binaries

We present measurements of the periods of dwarf nova oscillations (DNOs) and quasi-periodic oscillations (QPOs) in cataclysmic variable stars (CVs), many culled from published literature, but also others newly observed (in VZ Pyx, CR Boo, OY Car, Z Cha, AQ Eri, TU Men, HX Peg, CN Ori, V893 Sco, WX Hyi and EC2117−54). These provide data for 26 systems. We show that in general P_QPO∼ 15 P_DNO and that the correlation for CVs extends by three orders of magnitude lower in frequency the similar relationship found for X-ray binaries. In addition, we have found that there is a second type of DNO, previously overlooked, which have periods ∼4 times those of the regular DNOs (as well as those mined from publications, we have observed them in VW Hyi, OY Car, AQ Eri, V803 Cen, CR Boo, VZ Pyx, HX Peg and EC2117−54). Often both types of DNO coexist. Unlike the standard DNOs, the periods of the new type, which we refer to as longer-period DNOs (lpDNOs), are relatively insensitive to accretion luminosity and can even appear in quiescence of dwarf novae. We interpret them as magnetically channelled accretion on to the differentially rotating main body of the white dwarf primary, rather than on to a rapidly slipping equatorial belt as in the case of the standard DNOs. This is supported by published measurements of v sin i for some of the primaries. Some similarities of the DNOs, lpDNOs and QPOs in CVs to the three types of QPO in X-ray binaries (burst pulsations, high- and low-frequency QPOs) are noted.