GK Per, a classical nova system that erupted in 1901, is one of the more unusual examples of its type. It has the longest known orbital period for a classical nova (1.997d; Crampton, Cowley & Fisher 1986); and it contains a white dwarf primary with an evolved K2 sub-giant secondary. Most remarkably, the IRAS Sky Survey (1991) reveals that GK Per exhibits far-IR dust emission extending ~ 17′ to the NW and SE of the nova (Bode et al. 1987; Seaquist et al. 1989). We have re-analysed the IRAS data using maximum entropy reconstruction (Bontekoe et al. 1991; Bontekoe, Koper & Kester 1994) to resolve structures at a spatial resolution approaching the diffraction limit of IRAS, which is 1′ at 60 μm and 1.7′ at 100/μm.

The lowest mass white dwarfs that can have been produced by single star evolution in the lifetime of the Galaxy have masses of about 0.53 M⊙. There are however several white dwarfs known with significantly lower masses. Evolution in a binary provides a straightforward explanation as the star can lose its envelope before ever burning helium. The products are expected to be short-period binary stars. To test this we have looked at 15 such stars and have found that 8 of them are close binaries. Thus binary evolution does have a major role in the formation of low-mass white dwarfs. We discuss whether the non-detections could also be binary stars.

We are looking for stellar companions to very hot helium-rich subluminous (sdO) stars by using infrared photometry, optical CCD imaging and spectroscopy. The work is complemented with a comprehensive bibliographic and space-mission databanks search for the objects in our sample (Thejll, Ulla & MacDonald 1995). Companions can be used to indirectly measure the distance to the hot subdwarfs, and give clues about possible binary evolution scenarios. We have therefore gathered and analysed data for 57 hot subdwarfs in a search for IR excesses and companions. 43 of the stars are observed for the first time in the JHK bands and 28 objects are found to have larger IR fluxes than expected from a single sdO.

BE UMa is a close binary star, not transferring mass, with an extremely hot primary star irradiating the inner face of the cool secondary star. The light curve shows a large-amplitude, sinusoidal variation with a period of 2.29 d, and an eclipse that is centered on the minimum of the variation [1], [3]. According to [1], the eclipse is partial, not total. However, it has been argued [2] that the eclipse was really flat bottomed and thus total. This has important repercussions for the deduced model of the system. To resolve this issue we obtained simultaneous UBVR photometry of BE UMa using the Stiening 4-channel, high-speed photometer on the 82-inch telescope at McDonald Observatory. The mean light curves are shown in Fig. 1. The eclipse in all colours is round bottomed and partial. The different depths are caused by the different contribution from the red secondary star in each bandpass.

We present numerical calculations that simulate the evolution of a low mass (1M⊙) star transfering mass to a compact object (Muslimov & Sarna 1993; Ergma & Sarna 1996). Mass transfer starts when the secondary turns off the main sequence (having a small helium core). We have calculated 14 evolutionary sequences with the assumption of non-conservative or conservative evolution. We can conclude that near the bifurcation point the evolution is very sensitive to: (i) the assumption of conservative or non-conservative evolution, (ii) the structure of the mass losing star. Small changes in the initial period when the secondary fills its Roche lobe will lead to large changes in the final period and final mass of the remnant. Presently there are 40 known low-mass binary pulsars (LMBP). The evolutionary scenario for the wider systems (10 < Porb(d) < 1000) is connected with that of wide low-mass X-ray binaries (LMXB) in which the donor star will fill its Roche lobe after helium core formation.

Using population synthesis techniques, we investigate the orbital period distribution of ‘recycled’ pulsars. We find that the relative distribution of systems with periods between approximately one and fifty days are reasonably well reproduced. However, the existence of binary pulsars with periods between 70 and ~ 1000 d cannot be explained within the evolutionary framework considered here.