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ASASSN 17-fp shows 51-minute periodic variations

ATel #10354; Tom Marsh (University of Warwick), Steven Parsons (University of Sheffield), Vik Dhillon (University of Sheffield)
on 6 May 2017; 19:47 UT
Credential Certification: Tom Marsh (t.r.marsh@warwick.ac.uk)

Subjects: Optical, Request for Observations, Binary, Cataclysmic Variable, Transient

ASASSN-17fp (RA = 18 08 51.10, Dec = -73 04 04.2, J2000) was discovered as a transient source of V=16.2 at 2017 Apr 28.23 by the All Sky Automated Survey for SuperNovae ASAS-SN (Shappee et al. 2014, ApJ, 788, 48 and http://www.astronomy.ohio-state.edu/~assassin/index.shtml ). It was subsequently classified as a potential AM CVn star by ePESSTO (ATEL #10334).

We took high-speed multi-colour ugr photometry of ASASSN-17fp over the period 2017 May 06.095 to May 06.240 using the ESO New Technology Telescope at La Silla and the high-speed CCD camera ULTRACAM (Dhillon et al 2007, MNRAS, 378, 825). We used exposure times of 5 seconds in g and r and 10 seconds in u. We used a nearby star listed in the AAVSO Photometric All-Sky Survey (APASS) to determine g and r magnitudes (no corresponding u magnitude was available).

During our observations ASASSN-17fp had magnitude g = 16.03 +/- 0.02, r = 16.26 +/- 0.02 (1 sigma, inclusive of real variability), with g-r = -0.229 +/- 0.015. All three bands show the same periodic variation on a period of 51.0 +/- 0.1 minutes with amplitude 0.02 magnitudes on top of a trend with the system fading at a rate of 0.01 magnitudes per hour. Both the colour and period are typical of the colours and orbital periods AM CVn stars. The period (which may reflect tidally-induced distortion of the disc, but should still be close to the orbital period), is at the long period end of the AM CVn period distribution, suggesting that outbursts of ASASSN-17fp could be rare. Further coverage of the present outburst while ASASSN-17fp remains bright is strongly encouraged.