Unusual Cataclysmic Variable discovered by MASTER

ATel #5240; D. Denisenko, V. Lipunov, E. Gorbovskoy, P. Balanutsa, N. Tiurina, V. Kornilov, A. Belinski, N. Shatskiy, V. Chazov, A. Kuznetsov, V. Yecheistov (Moscow State University, SAI), V. Yurkov, Y. Sergienko, D. Varda, E. Sinyakov, A. Gabovich (Blagoveshchensk Educational University), K. Ivanov, S. Yazev, N. Budnev, E. Konstantinov, O. Chuvalaev, V. Poleshchuk, O. Gress, A. Frolova (Irkutsk State University), V. Krushinsky, I. Zalozhnih, A. Popov, A. Bourdanov (Ural Federal University), A. Parkhomenko, A. Tlatov, D. Dormidontov, V. Senik (Kislovodsk solar station of the Pulkovo observatory RAS), P. Podvorotny, V. Shumkov, S. Shurpakov (MASTER team members), H. Levato, C. Saffe (ICATE), C. Mallamaci, C. Lopez and F. Podest (OAFA)
on 31 Jul 2013; 21:39 UT
Credential Certification: Vladimir Lipunov (lipunov2007@gmail.com)

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MASTER OT J204200.48+041839.9 - new bright CV

MASTER-Amur auto-detection system discovered OT source at (RA, Dec) = 20h 42m 00.48s +04d 18m 39.9s on 2013-07-31.59277 UT. The OT unfiltered magnitude is 15.8m (limit 18.7m). The OT is seen in 6 images. There is no minor planet at this place. We have reference image without OT on 2013-04-18.81102 UT with unfiltered magnitude limit 17.0m.

The new object is identical to the blue star USNO-A2.0 0900-18910404 (20 42 00.42 +04 18 40.3 R=16.3 B=16.0) = USNO-B1.0 0943-0535946 (20 42 00.425 +04 18 40.02 B1=16.86 R1=16.69 B2=17.28 R2=19.21 I=15.63). It is showing the large variability between red Palomar plates obtained on 1951-08-09 and 1991-09-12. Comparison of the 1st and 2nd epoch POSS red plates is uploaded to http://master.sai.msu.ru/static/OT/J204200+041839-DSS-Red.jpg (4'x4' FOV).

The OT is present in Sloan Digital Sky Survey as SDSS J204200.44+041839.9 with r=16.63, likely observed in bright state. It has an UV counterpart GALEX J204200.4+041840 (FUV=16.97+/-0.03, NUV=16.90+/-0.01). However, nothing is present at this position in 1RXS catalogue. There is no radio detection. No variable stars are found within 10' radius in AAVSO VSX and GCVS.

This area of sky in Delphinus was observed by NEAT project with 76 images obtained on 27 nights from 1997 Aug. 02 to 2005 Sep. 02. The variable object remained at the bright level on 2001 June 03, 20, 25, 29, 30, Aug. 20 and Sep. 22. Such a long brightening is inconsistent with the dwarf nova outburst and can be interpreted as either a VY Scl (anti-nova) or AM Her (polar) variable in a high state. Catalina Real-time Transient Survey data from CSS and MLS show the variability from ~19.5 to 15.9m. CRTS light curve also does not conform to the dwarf nova behavior.

If the object were a polar (magnetic CV), one should have expected the X-ray detection by ROSAT which is not the case. Thus, the most likely hypothesis for MASTER OT J204200.48+041839.9 is that of VY Scl type variable. However, VY-type CVs have typical orbital periods of ~4 hr or larger and late K-class secondary components. That, in turn, contradicts to the SDSS colors for SDSS J204200.44+041839.9 with u=16.79, g=16.45, r=16.63, i=16.78, z=16.91. Large negative (g-r) and (r-i) color indices are more typical for the extremely short-period cataclysmic variables with low-mass donor of spectral class M6 and later. The object appears to be unusual and requires a further study.

Follow up spectral and photometric observations are strongly encouraged. The discovery and reference images are available at: http://master.sai.msu.ru/static/OT/204200.48041839.9.png

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