Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables – I. Observations of VW Hyi

From archived and recent high-speed photometry of VW Hyi we find dwarf nova oscillations (DNOs) occasionally present throughout outburst, evolving from a 14.06-s period at maximum to >40 s near the end of outburst. A relatively slow increase of period is followed by a rapid increase and a subsequent decrease.
Quasi-periodic oscillations (QPOs) are seen at periods of hundreds of seconds. For the first time, the evolution of a QPO period is seen, increasing steadily during the final decline of an outburst. The occasional presence of two DNOs, separated in frequency by the QPO frequency, suggests reprocessing of the rotating DNO beam by a 'wall' rotating progradely in the disc at the QPO period.     

Observations of Hipparcos short-period red giant stars

Optical UBV(RI) _C and infrared JHK photometry is presented of a small sample of giant stars with short periods in the Hipparcos catalogue. Observations were limited, but were sufficient to rule out most of the Hipparcos periods. Radial velocity measurement were also made for a few stars, over six successive nights. Low-level variability was detected in a few stars. It is argued that in most cases the brightness variations are primarily due to temperature changes. These findings show that high-overtone pulsations in M giant stars occur, if at all, in a far more limited number of stars than proposed in the authors' previous discussion of the Hipparcos data alone.     

High-speed energy-resolved STJ photometry of the eclipsing binary UZ For

We present high-time-resolution optical photometry of the eclipsing binary UZ For using a superconducting tunnel junction (STJ) device, a photon-counting array detector with intrinsic energy resolution. Three eclipses of the ∼18-mag 126.5-min orbital binary were observed using a 6×6 array of tantalum STJs at the 4.2-m William Herschel Telescope on La Palma. The detector presently provides individual photon arrival-time accuracy to about 5 μs, and a wavelength resolution of about 60 nm at 500 nm, with each array element capable of counting up to ∼5000 photon s⁻¹. The data allow us to place accurate constraints on the accretion geometry from our time- and spectrally resolved monitoring, especially of the eclipse ingress and egress. We find that there are two small accretion regions, located close to the poles of the white dwarf. The positions of these are accurately constrained, and show little movement from eclipse to eclipse, even over a number of years. The colour of the emission from the two regions appears similar, although their X-ray properties are known to be significantly different: we argue that the usual accretion shock may be absent at the non-X-ray-emitting region, and instead the flow here interacts directly with the white dwarf surface; alternatively, a special grazing occultation of this region is required. There is no evidence for any quasi-periodic oscillations on time-scales of the order of seconds, consistent with relatively stable cyclotron cooling in each accretion region.