COMMISSION 27 OF THE I.A.U.
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 1429

                                                     Konkoly Observatory
                                                     Budapest
                                                     1978 May 19


             ULTRAVIOLET FLARE IN FK Ser

In the course of analysing the data returned by the S2/68
ultraviolet sky-survey experiment aboard the TD-1 satellite in
1972-4, a systematic check for transient sources was undertaken.
All known transient sources (e.g., novae) observed at other 
wavelengths between February 1972 and May 1974 were specifically 
examined, along with known flare stars within 25 pc of the Sun
(Kunkel 1975). No previously observed transient phenomena were
confirmed in the ultraviolet, but one apparent flare not known
to have been observed elsewhere was recorded from the variable
star discovered by Stienon (1971) and designated FK Ser (Kukarkin
et al. 1976).
   In view of the sensitivity limits of the S2/68 instrument
(Boksenberg et al. 1973), there would have been little chance of
detecting FK Ser, whose mpg ranges from 10.9 (Hoffleit 1972) to
11.5 (Stienon 1971), save in the near-UV photometric channel A1
with FWHM 310 A centred on 2790 A. Indeed, no signal was detectable 
in the three spectrophotometric channels blueward of A1 in
any of the 14 orbital passes, 5 of which were in any case 
contaminated by the nearby 8m.50 A3 star HD 168392. Two of the 
remaining (unblended) scans, both with the target well-centred, manifest
a measurable signal in A1, a strong indisputable one on 27 September 
1972 (J.D. 2441588.46243) and a weak probable one on 25 March
1974 (J.D. 2442132.44088). The respective fluxes were 1.9 +- 0.2
and 0.4 +- 0.1 in units of 10^-12 erg cm^-2s^-1A^-1. One orbital 
revolution (95 min) later, the 1974 flare becomes lost in background
noise; the subsequent history of the 1972 flare is inconclusive
because the target is too far displaced by the next scan.
   The nature of FK Ser remains enigmatic: it has been likened
to the BY Draconis variables by Stienon (1971), Hidajat (1971),
and Chugainov (1974) although certain spectroscopic anomalies
(Herbig 1973) and the infrared excess (Zappala 1974, Hackwell et al.
1974) suggest that it may be a post-T Tauri star at least 3 mag above
the main sequence. Moreover, the conjecture that the nearby B2 IV-V
star HD 170740 and FK Ser were coeval, on the basis of their proper
motions which can be extrapolated back to a dark interstellar cloud,
lends additional weight to the latter interpretation. Balmer emission 
lines have been reported by Stienon (1971), Hidajat (1971),
MacConnell (1971), Herbig (1973), and Zappala (1974), while the latter 
two have also pointed out the strong. Li I line at 6707 A and
Ca II H and K emission lines. MacConnell (1971) first identified
FK Ser as BD -10d4662, but Herbig found it to be a close visual binary
of similar magnitude (Delta m=0m.5) and spectroscopic characteristics
(about K5p V for component A and K7p V for component B).
   It is quite conceivable that the ultraviolet variability is
greater than that in the visible on the following grounds: (1) the
"strong ultraviolet continuum" reported by Stienon (1971); (2) the
discrepancy between the (U-B) measures on two nights by N.K. Rao
(Herbig 1973) and a value by R.J. Brucato (Zappala 1974) despite
the agreement among all three (B-V) measures; and (3) the absence
of infrared variability over timescales >~ known timescales of 
variability at shorter wavelenghts. Nevertheless, it would be instructive
to examine any visible or infrared observations made simultaneously
or contemporaneously with the two S2/68 observations, which are
thought to represent the first mid-UV record of a stellar flare. Any
such observations are earnestly solicited.


                                    JON DARIUS

                                    University of London Observatory
                                    Mill Hill Park
                                    London NW7 2QS, England



References:

Boksenberg, A., et al. 1973, Monthly Notices Roy. Astron.Soc. 163, 291 [BIBCODE 1973MNRAS.163..291B ]
Chugainov, P.F. 1974, Izv. Krymsk. Astrofiz. Obs. 52, 3 [BIBCODE 1974IzKry..52....3C ]
Hackwell, J.A., Bopp, B.W., and Gehrz, R.D. 1974, Astrophys. J. 192, L79 [BIBCODE 1974ApJ...192L..79H ]
Herbig, G.H. 1973, Astrophys. J. 182, 129 [BIBCODE 1973ApJ...182..129H ]
Hidajat, B. 1971, I.B.V.S. No. 559
Hoffleit, D. 1972, I.B.V.S. No. 724
Kukarkin, B.V., et al. 1976, Third Suppl. to the Third Edition of the
   G.C.V.S., Nauka, Moscow [BIBCODE 1976tstt.book.....K ]
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   15, D. Reidel, Dordrecht-Holland [BIBCODE 1975IAUS...67...15K ]
MacConnell, D.J. 1971, I.B.V.S. No. 570
Stienon, F.M. 1971, I.B.V.S. No. 545
Zappala, R.R. 1974, Astrophys. J. 187, 257 [BIBCODE 1974ApJ...187..257Z ]