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Discovery of a variable star with evident peculiarities of the light curve in the field of Selected Areas 95

ATel #11279; Mannucci M. (Osservatorio Astronomico Margherita Hack OAMH, Italy), Santangelo M. M.M. (Osservatorio Astronomico di Capannori OAC, Italy), Montigiani N. (Osservatorio Astronomico Margherita Hack OAMH, Italy)
on 9 Feb 2018; 15:21 UT
Credential Certification: Filippo Mannucci (filippo@arcetri.astro.it)

Subjects: Optical, Variables

We report the discovery of the photometric variability of GSC 66-869 in SA 95. Its J2000 coordinates from GSC are: R.A. = 03h 53m 17.72s Decl. = +00° 04' 22.4", about 3' NW of the variable FASTT89 = SA 95-107 whose periodicity we detected too. GSC 66-869 has V = 13.6 from APASS, and its amplitude from our data is about 0.11 mag. We discovered its variability by means of 2751 unfiltered CCD frames taken in a range of 63 days with the 0.25-m f/10 telescope of OAMH. Differential CCD aperture photometry was performed using Landolt's standard SA95-101 in the same field. GSC 66-869, although having measurements in ASAS-SN, CSS, NSVS and TASS, is not listed as variable in the following catalogs: GCVS, AAVSO-VSX, ASAS-SN, Catalina sky surveys, Linear survey, NSVS, SDSS stripe 82, ASAS, TASS, MOTESS-GNAT, SuperWASP, DIRBE and on many IBVS and OEJV. Our time series shows a smooth intra-night variability with some dips, and a peculiar inter-nights variability with minima (apart from dips) switching state from deeper and sinusoidal to less deep and flat. We have analyzed our time series of GSC 66-869 with the software Period 04, PerSea 2.6 and ATSA (Santangelo et al., 2007, AN 328, 55) using many algorithms. We have found a periodicity P1 = 0.67254 ± 0.00007 d with p-value << 0.001, but our data can be very well phased by the P2 = 2*P1 = 1.34508 d period too. We have found confirmation of the same two alternative periods from our time series analysis of ASAS-SN data. We have not found significant periodicities in CSS data. In NSVS data there could be a marginal detection of a 1-day alias of 1/P1. If the period is P1, there is one dip on the rising branch of our phased light curve of GSC 66-869 and this dip has shape, duration and depth very similar to the transient flux dips in EPIC 205046529B or in RIK 210; according to Stauffer et al. (2017 AJ 153, 152) these dips could be due to orbiting clouds of material. Our alternative hypotheses for the dip are a disk, or rings of debris, or streams, or the transit of a dark body like a brown dwarf. From our data we derived a linear time-shift (about 1.7 minutes/day) of the time of the minimum of the dip. If the period is P1, apart from the dip, the light curve shows the above mentioned switching states of minima, with a slightly longer rise and shorter fall of the light curve resembling some peculiar radial pulsators like V2551 Cyg (Kjurkchieva et al. 2017, RAA 17, 69) or some peculiar HADS or RRc. If the period is P1 with the flat minimum state, then the light curve resembles that of the DSCT V116 in NGC 6791 (Hartman et al. 2005, AJ 130, 2241), but a period of 0.67254 d is too long for a DSCT. If the period is P2, then the light curve strongly resembles that of an EB or EW eclipsing binary with the peculiarities of two dips shifted each other by 0.5 phases and the primary minimum that shows a change in its depth; on the early nights of our observed time span the primary minimum was deeper and sinusoidal, but on subsequent nights it suddenly changed to very flat (constant within 0.01 magnitudes) and with the same depth of the secondary minimum. If the period is P2, then the state with the deep sinusoidal minimum resembles an EB lightcurve, instead the state with the flat minimum resembles an EW lightcurve of an extreme mass ratio binary (EMRB) like V857 Her (Qian et al. 2005, AJ 130, 1206) perhaps with a very small q parameter. A period of 1.34508 d is still border-line for an EW, but it is fully compatible with an EB or RS. From APASS BVg'r'i' and 2MASS JHK we got the observed colour indexes; e.g. (B-V)=0.937. From NED database we got Av = 0.786; so, if the ratio of total to selective extinction is 3.1, then E(B-V) = 0.254; so (B-V)o = 0.683. No spectra of GSC 66-869 are available from SDSS. The spectral class estimated from colour indexes corrected for interstellar extinction is G5 V or G1 III, so it would be compatible with an EB or EW, but not with a DSCT nor a RRc.