Discovery of another eclipsing binary variable star in the field of the exoplanet host star WASP 26

ATel #10884; Montigiani N., Mannucci M. (Osservatorio Astronomico Margherita Hack - OAMH, Italy), Santangelo M. M.M. (Osservatorio Astronomico di Capannori - OAC, Italy)
on 23 Oct 2017; 19:47 UT
Credential Certification: Filippo Mannucci (filippo@arcetri.astro.it)

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We report the discovery of a previously unknown eclipsing binary in the field of the exoplanet host star WASP 26 on 633 unfiltered CCD frames taken with the 0.25 m f/10 Schmidt-Cassegrain telescope of OAMH. We have cross identified this 12.7 R magnitude (and about 0.1 magnitudes amplitude) new variable star with GSC 5839-905 = USNO B1 0747-0003770 = NSVS 14646936 = CSS J001749.0-151349 = 2MASS J00174895-1513492 = GalexAsc J001748.92-151348.6 in Cetus constellation. Its coordinates (J2000) from GSC are 00:17:49.00 -15:13:48.2. This star is not listed in the Hipparcos or Tycho catalogues. We performed differential CCD aperture photometry on GSC 5839-905 using two field stars as comparison and check. We have checked for previously known variable stars at this position on the GCVS (both printed and on-line versions), AAVSO-VSX (on-line), Catalina sky survey (both CS_DR1 and CS_DR2) both printed (Drake et al. 2014, ApJS 213,9; Drake et al. 2017, MNRAS 469, 3688) and on-line, Linear survey (Palaversa et al., 2013, IBVS 6065), NSVS both printed (Hoffman et al. 2009, AJ 138, 466) and on-line, ASAS (on-line), TASS (Droege et al. 2006, PASP 118,1666; Richmond 2007, PASP 119, 1083), SuperWASP, DIRBE and on various numbers of IBVS and OEJV; but this star, although listed in NSVS and CSS, is not known as variable. Our time series of GSC 5839-905 has been analyzed with three different software: Period 04 (Lenz & Breger, 2004, IAUS 224, 768), PerSea 2.6 (Maciejewski, 2007), and ATSA (Santangelo et al., 2007, AN 328, 55). Period 04 implements DFT + least-squares fit of sinusoids, PerSea implements the Periodic Orthogonal Polynomials (POP) method by Schwarzenberg-Czerny sometimes referred to as AoV, and ATSA implements tens of algorithms (among them: DFT, POP, Vanicek, DCDFT, Lomb-Scargle, residualgram, PDM, SLM, SVM, ANN, and many more + least-squares fit of sinusoids) all used in this analysis. We have found a periodicity P1 = 0.330994 ± 0.000012 d (with p-value << 0.001), corresponding to the frequency f1 = 3.0212 c/d, and the light curve of this star can be modeled as the sum of this f1 frequency plus a 2*f1 = 6.0424 c/d, or perhaps a one-day alias of these frequencies; the light curve is that of an eclipsing binary of W-Uma type (EW) and W-subtype. The frequency f1 stands out clearly especially with the POP and residualgram algorithms with two harmonics. From the 2MASS survey we get J-H = +0.313 and H-Ks = +0.08 for this star which, given an almost negligible interstellar extinction due to its high galactic latitude (see the NED extinction calculator based on Schlafly & Finkbeiner 2011 maps), gives a probable spectral class of G2 V or G0 III (according to the calibration of Bessell & Brett, 1988) or G5 V (according to that of Mamajek, 2016). The period, amplitude, and spectral class are perfectly compatible with the EW-type eclipsing binary shape of the light curve and especially with a W sub-type (Sterken & Jaschek, 1996, Light curves of variable stars, Cambridge univ.press). An alternative model with a single frequency f1 = 6.0381 c/d and DSCT light curve is by far less well supported by our data (larger spread of the points around the modeled light curve and larger chi-square of the fit). The 60 data points given by NSVS on this star have large errors, so it is not possible to derive a significant period from them. On some of our CCD frames we have re-discovered also a previously very poorly known V=14.98 magnitude eclipsing binary variable star recently discovered by Southworth et al. (2014, MNRAS 444, 776) in the same field of WASP 26 and known as 2MASS J00182645-1511492. We add another minimum of its light curve to the two discovered by them.