COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2252 Konkoly Observatory Budapest 1982 December 20 HU ISSN 0374-0676 FURTHER OBSERVATIONS OF EV Lac IN 1974 High-speed photometric techniques developed for observations of optical pulses and ultra-rapid variability on time scales of 1 millisecond to 10 seconds were utilized for the patrol of the dMe flare star, EV Lac, in 1974. The 36-inch Yapp reflector at the Royal Greenwich Observatory was employed with the 2-star photometer, designed by Dr Bingham, to simultaneously monitor EV Lac and a comparison star, BD +43deg4310 (Sp K0, 22h46m26s, +44deg25.6', 1950), separated 28.2' from EV Lac. A multi-channel analyzer (Fabri-Tek Model 1072) was fed with the amplified dual pulses from the ultraviolet photoelectrons detected at two uncooled EMI 6256A photomultipliers. Additional observations on three other nights were made using the Brookdeal 501 Photon-counting System and the data logger previously employed at the RGO (See Andrews 1973). On 19 August 1974 four runs each lasting 4096 seconds were made with the Fabri-Tek System. The progress of the dual counting for the two stars in the standard U-band filter was displayed on an oscilloscope and recorded on punch tape. A dwell time of 8 seconds for each channel was used. During the third run, a stellar flare of amplitude 0.3mag in the ultraviolet with duration 14 minutes was observed against the combined light of EV Lac and its faint companion (See Andrews and Chugainov 1969). The time of maximum was 23h05m45s UT. However, due to a malfunction in the tape punch the individual data were irretrievable. The only record retained was on a tracing onto transparent paper from the oscilloscope display, and a detailed diary of events. On 20-22 August 1974 observations were continued using the Brookdeal System. Continuous monitoring in the ultraviolet in the auto-repeat mode was performed with a 10-second integration. The counter was maintained at a fairly constant temperature in an enclosure within the dome. The 13 hours total coverage of EV Lac is given in Table I with Universal Time noted to the nearest minute. Parentheses indicate poor sky Table I Coverage (UT) of EV Lac in the Ultraviolet 1974 19 Aug (2052-2102,2103-20),2121-50,2154-2203,2211-41,2242-2319,2324-29 2336-2400. 20 Aug 0000-05,0006-33,0053-58,0216-30,(0230-0311),(2050-2103,2119-24), (2127-29),2140-54,2158-59,2206-55,2326-2400. 21 Aug 0000-23,0033-38,0050-0101,0113-23,0125-29,0148-53,0155-0203, (0205-08,0210-19),0255-0300,(2024-43),2054-2109,2115-28,(2128-33), 2146-57,2219-21,2127-29,2241-45,2302-51. 22 Aug (0008-19,0020-39),0041-0105,0114-16,0118-19,0134-41,(0241-49), (0259-0302),(2047-2100),2109-28,2146-53,2256-2321,2325-2400, 23 Aug 0000-06,0008-22,0052-0111,0121-55,0209-22,0243-0308. TOTAL COVERAGE 13h08m over 4 consecutive nights. conditions or observations made during semi-twilight (Astronomical Twilight : End 21h25m, Begin 02h40m UT). A total of 3210 dual counts were successfully recorded on the data logger under good sky conditions. See Fig. 1, which [FIGURE 1] shows the mean count-rates over 1 minute intervals. Two partially-recorded flares were noted during 21 August (Nos. 2 & 3), but fluctuations at 00h12m UT on the same night are thought to be noise. Flare No. 5 on 22-23 August showed structure on its decline (a,b in Fig. 1). Details of all flares are summarized in Table II. The r.m.s. scatter in the count rate is indicated Table II Observed Flares of EV Lac No. 1974 UT(Max) DeltaU Duration I_0+f / I0 P (mins) (mins) 1 19 Aug 23h05m45s ~ 0.3mag ~ 14 - - 2 21 Aug (21 12 )* - - - - 3 21 Aug (22 40 )* > 0.4 > 4 - - 4 22 Aug 23 03 00 0.66 31 1.84 4.71 5 23 Aug 01 00 26 0.41 16 1.46 4.72 * Incomplete light curves. Rise or maximum missed. at intervals throughout the night in Fig. 1, and the vertical curly lines indicate shifts in the zero point for the two stars. Observations were all made at small air mass. See top scale in Fig. 1. The well-observed flare on 22-23 August (No. 4) was a typical, double- peaked event with a maximum amplitude of 0.66mag, and is shown in detail in Fig. 2. Since the author had limited means to follow the quiescent light [FIGURE 2] during the run using the data logger, the slow change apparent in Fig. 1, amounting to a total rise of about 0.3mag only became evident during reductions. Flare No. 4 occurred superimposed on this gradual rise in the ultraviolet. The second flare on that night two hours later was very slow with a rise time of about 8 minutes and an amplitude above "quiescence" of 0.41mag. The energy considerations for this night, however, are open to doubt due to the very small signal-to-noise ratio in the ultraviolet. The total data has not been searched for flare events on very short time scales. A. D. ANDREWS Armagh Observatory Armagh BT61 9DG Northern Ireland References : Andrews, A.D., 1973. IBVS 851. Andrews, A.D. & Chugainov, P.F., 1969. IBVS 370. (For details on the comparison star, BD +43deg4310, see I.B.V.S. No. 2253)