Discovery of photometric variability of two new EA-type eclipsing binary stars with sharp minima

ATel #12797; Montigiani N. (OAMH - Osservatorio Astronomico Margherita Hack, Firenze, Italy), Santangelo M. M.M. (OAC - Osservatorio Astronomico di Capannori, Lucca, Italy), Mannucci M. (OAMH - Osservatorio Astronomico Margherita Hack, Firenze, Italy)
on 23 May 2019; 01:17 UT
Credential Certification: Filippo Mannucci (filippo@arcetri.astro.it)

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We report the discovery of the photometric variability of two new EA-type variable stars with sharp minima: both of them lie in the same field, at an angular distance of about 9' each other. We cross-identify them as UCAC4 511-031703 = GSC2.2 N22330034352 and UCAC4 510-032641 = GSC2.2 N22330015679. UCAC4 511-031703 has J2000 coordinates R.A. = 06h53m12.806s Decl. = +12d03p31.86s, V = 14.73 from APASS; from our data its range is about 0.58 magnitudes. UCAC4 510-032641 has J2000 coordinates R.A. = 06h53m39.414s Decl. = +11d57p28.13s, V = 14.95 from APASS; from our data its range is about 0.47 magnitudes. We discovered the photometric variability of both stars by means respectively of 2021 and 2023 unfiltered CCD frames taken in 23 nights in a time span of 82 days with a SBIG ST-10XME CCD camera at the 0.35-m f/10 Schmidt-Cassegrain of OAMH. Differential CCD synthetic aperture photometry of UCAC4 511-031703 was performed using USNO A2.0 0975-04120682, USNO A2.0 0975-04112939 and USNO A2.0 0975-04108994 as comparison and check stars. Differential CCD synthetic aperture photometry of UCAC4 510-032641 was performed using USNO A2.0 0975-04135392 and USNO A2.0 0975-04137526 as comparison and check stars. UCAC4 511-031703 and UCAC4 510-032641 are not listed as variable star in the following catalogs and surveys: GCVS, NSV, AAVSO-VSX, ASAS-SN, Catalina, Linear, PanSTARRS DR2, Gaia DR2, NSVS, SDSS, ASAS, TASS, MOTESS-GNAT, SuperWASP, and on IBVS from 5500 to 6200 and on many OEJV. We have analyzed our time series of UCAC4 511-031703 and UCAC4 510-032641 with the software PerSea 2.6 and 2.7, ATSA (Santangelo et al., 2007, AN 328, 55), and Period 04. Due to the shape and sharpness of the minima in the phase diagrams many algorithms such Fourier and least squares ones were fooled; so in this case the useful algorithms have proven themselves to be the string length ones, the phase binning ones with many bins (with ATSA software), and Schwarzenberg-Czerny's POP and AoVTr with many harmonics (with ATSA and PerSea software). With ATSA (Penfold's string length method, Stellingwerf's PDM, and POP) and PerSea (POP and AoVTr) applied to our photometric data of both stars we have found the following periods: for UCAC4 511-031703 P = 4.205 ± 0.005 d and for UCAC4 510-032641 P = 6.545 ± 0.006 d. The different algorithms and software used are in very good agreement for each star. We got an uncertain detection of a period of 4.206 d in 1149 ASAS-SN data of UCAC4 511-031703. We have not detected a periodicity in 849 ASAS-SN data of UCAC4 510-032641, probably due to inadequate data sampling. For UCAC4 511-031703, from APASS we get B-V=+0.401, and from NED (based on Schlafly's maps) we get Av=0.505; so if the total to selective extinction ratio is 3.1 we obtain E(B-V) = +0.163 from which we derive (B-V)o = +0.238 which suggests a spectral class around A7 III. For UCAC4 510-032641, from APASS we get B-V=+0.855, and from NED we get Av=0.536; so if the total to selective extinction ratio is 3.1 we obtain E(B-V) = +0.173 from which we derive (B-V)o = +0.682 which suggests a spectral class around G4 IV. So we wonder if this last binary star could be a cool Algol or an RS Cvn-type variable star with an EA-type light curve. For both these two new variable stars the light curve shapes, periods, amplitudes and possible spectral classes derived from (B-V)o are compatible with EA-type eclipsing binary stars with sharp minima. If we call Delta(phi)min1 the phase span (width) of the primary minimum and Delta(phi)min2 the phase span of the secondary minimum in the phase diagram then we get the following values. For UCAC4 511-031703 Delta(phi)min1 = 0.057 and Delta(phi)min2 = 0.054. For UCAC4 510-032641 Delta(phi)min1 = Delta(phi)min2 = 0.032. We have made a (not complete) search in the scientific literature among some hundreds of EA stars for light curves with Delta(phi)min 1 and 2 sharper than 0.032, and we have found very few such variables.