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Astron. Astrophys. 347, 203-211 (1999)

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2. Observational histories

2.1. YY Draconis

The 16th magnitude star YY Dra was discovered in 1934 and originally mis-classified as an Algol-like system. Detected as an X-ray source by Ariel V (3A 1148+719) and Einstein (2E 1140.7+7158), it was subsequently reclassified as a cataclysmic variable (see the discussions in Patterson et al. 1992 and Patterson & Szkody 1993 for the history of this source). Optical radial velocity observations by Friend et al. (1988) revealed its orbital period as 3.97 hr, whilst time resolved infrared spectroscopy by Mateo et al. (1991) yielded estimates of other system parameters. The intermediate polar nature of YY Dra was eventually recognised by Patterson et al. (1992), who discovered optical photometric modulation at periods of about 265 s and 275 s. Indications of sub-harmonics at periods of 529 s and 550 s suggested that the spin period of the white dwarf might actually be 529 s, and that 550 s was the beat period between the orbital and spin periods (ie. the spin period of the white dwarf in the binary reference frame). The dominant signals at the shorter periods indicated that the pulse profile is `double-peaked'.

A series of short X-ray observations of YY Dra with the ROSAT PSPC and HRI (Patterson & Szkody 1993; Beuermann & Thomas 1993; Reinsch et al. 1995) confirmed the earlier findings and revealed the following periods: 529.2 s (1990, PSPC all sky survey); 265 s (1991, PSPC); 264 s (1992, HRI); 264.6 s and 530.9 s (1993, PSPC). These authors concluded that the true spin period of the white dwarf was around 530 s, that the presence of a dominant signal at half this period indicates that the system usually accretes onto both poles of the white dwarf, and that the emission sites are nearly identical.

Subsequent Hubble Space Telescope spectroscopy of YY Dra (Haswell et al. 1997) established a more accurate white dwarf spin period of 529.31 s, by combining their detection of UV pulsations at a period of 264.71 s with the previous detections referred to above. A beat period of 273 s was also detected and attributed to reprocessing of the X-ray pulse in a structure fixed in the orbital frame. Haswell et al. determined a precise orbital period of 3.968976 hr and calculated the system parameters, including an inclination angle of 45o.

2.2. V709 Cassiopeiae

This source was recognised as an intermediate polar by Haberl & Motch (1995), following its detection in the ROSAT All Sky Survey as RX J0028.8+5917. A follow up 18 ksec pointed observation with the ROSAT PSPC revealed a pulse period of 312.8 s and a conventional `hard' intermediate polar X-ray spectrum. Motch et al. (1996) subsequently noted that RX J0028.8+5917 was probably coincident with previously catalogued sources detected by HEAO-1 (1H 0025+588), Uhuru (4U 0027+59) and Ariel V (3A 0026+593), and identified the X-ray source with a 14th magnitude blue star, V709 Cas. The optical spectra of this star show radial velocity variations with periods of either 5.4 h or 4.45 h, the two being one day aliases of each other (Motch et al. 1996). One of these periods is assumed to be the orbital period of the system.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999
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