Evidence of abrupt changes in the orbital periods of

2024 February 8

David Boyd

We report evidence from eclipse timing that two otherwise apparently normal cataclysmic variables have experienced abrupt changes in their orbital periods. The orbital period of HS 2325+8205 increased by 22.15ms (1.3×10^–6 of the orbital period) around 2011 Feb 1 and the orbital period of EP Dra increased by 4.51ms (7.2×10^–7 of the orbital period) around 2001 Dec 7. In neither case was there any apparent change in the subsequent behaviour of the system. These changes are currently unexplained.

Introduction

Cataclysmic variables (CVs) are binary systems in which a white dwarf (WD) accretes hydrogen-rich material from a main sequence (MS) companion via Roche lobe overflow through the inner Lagrangian point (L1). The accreting stream of material forms a disc around the WD before eventually settling onto its surface. The gradual accumulation of material in the disc may eventually lead to thermal instability which causes the disc to brighten for a period before fading back to its quiescent state. These bright events are known as outbursts and are a signature of the sub-class of CVs known as dwarf novae (DNe). If the WD has a magnetic field, formation of a disc may be partially inhibited or, if the magnetic field is sufficiently strong, prevented altogether, in which case accretion flows directly onto one or both magnetic poles of the WD. These are known as AM Her stars or polars.

HS 2325+8205

First reported as variable by Morgenroth (1936),¹ HS 2325+8205 (hereafter HS 2325) was subsequently identified as a possible CV in the Hamburg Quasar Survey (Aungwerojwit et al., 2006).² In quiescence its magnitude is around 17, rising to slightly brighter than 14 in outburst. It experiences eclipses which are 0.4 magnitudes deep during outburst and around 1 magnitude deep in quiescence. The presence of eclipses indicates a relatively high orbital inclination. Its light curve shows no evidence of standstills between outburst and quiescence so it is unlikely that HS 2325 is of the Z Camelopardalis (UGZ) type of DN. Outbursts all appear to have very similar maximum brightness with no evidence of brighter superoutbursts, so it is unlikely to be an SU Ursae Majoris (UGSU)-type DN. We are therefore drawn to the conclusion that HS 2325 is a U Geminorum or SS Cygni (UGSS)-type DN (Hellier, 2001).³ HS 2325 was the subject of a paper by Pyrzas et al. (2012) which found an inclination of 75° and an orbital period of 4.6640263(3)h.⁴ (The number in parentheses indicates the uncertainty in the last digit.) This puts HS 2325 above the period gap and therefore likely to be experiencing magnetic braking.

Observations

Unfiltered photometry of HS 2325 from the 1.2m telescope at the Kryoneri Observatory in Greece between 2004 and 2006 was obtained from Prof Boris Gänsicke. These data were phased on the above orbital period and nine eclipses were identified. The first Kryoneri eclipse was used as T_zero for the ephemeris and given cycle number zero. To find the time of minimum for these and subsequent eclipses the lower half of each eclipse was fitted with a quadratic polynomial. Uncertainty on the time of minimum was derived from the uncertainties in the photometry measurements.

A total of 4,526 V-band and unfiltered magnitude measurements of HS 2325 by various observers between 2007 and 2021 were obtained from the American Association of Variable Star Observers (AAVSO) International Database (AID). A list of observers who contributed more than 10 measurements is given in Table 1. Measurements were obtained using a variety of telescopes up to 0.4m aperture. These data were searched for time series which recorded eclipses and could be analysed to provide a time of minimum. In total 18 such eclipses were found, all between 2007 and 2009. All but one of these were recorded by the late Ian Miller.

A further 6,208 V-band and unfiltered magnitude measurements were made by the author between 2009 and 2022 with a 0.35m telescope. Magnitudes were calculated by differential aperture photometry with respect to the magnitudes of comparison stars from the AAVSO chart for HS 2325. These provided a further 62 eclipse timings which are listed in Table 2. Subsequently, three eclipse timings between 2012 January and 2013 August were found in Hardy et al. (2017).⁵ This made a total of 92 eclipses with measured timings. The times of all eclipses were converted to Heliocentric Julian Date (HJD).

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