COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3344 Konkoly Observatory Budapest 23 June 1989 HU ISSN 0374 - 0676 Delta Sct like variability in the Am star 18 Vul In this short communication the discovery of luminosity variations in the A3mIII star 18 Vulpeculae (HR7711) is reported, on the basis of rapid photometric observations performed looking for periodicities ranging from a few minutes to several hours. 18 Vul is an already known magnetic variable star (Bertolini, Foschini and Piccioni, 1974) with a rotation period ~9.31d. The Stromgren photometry (Hauck and Mermilliod, 1980) provides values of b-y=0.047 and C_1=1.149 which place it near the blue border of the delta Sct instability strip. It is member, and a possible blue straggler (BS) star, of the stellar cluster NGC6882. 18 Vul was observed by Breger (1969) in a large search for delta Sct variables amongst stars within the instability strip. He observed it twice (for 0.4h and 3.5h long respectively) and concluded that the star was stable in the range of interest at a level of ~2 mmag. Manteiga and Martinez Roger (1988) observed it in a campaign to determine the IR colours of a large group of BS stars, finding a strange photometric behaviour; so it was decided to observe 18 Vul again carefully also in the visible range. In Table 1, we present the journal of the observations made in 3 consecutive nights in October 1988. Observations were performed through a B filter, using the IAC-UBV photometer (Belmonte, 1986), working in photon counting mode, attached to the Cassegrain focus of the 1.5m Carlos Sanchez Telescope (TCS) of the Observatorio del Teide (Tenerife, Spain). In the three nights, small light variations were evident in the light curves. The data were reduced via the standard fit to airmass, obtaining the residual data series, the standard deviation of the fit (sigma) and the extinction coefficient (K_B) amongst other parameters. Both sigma and K_B are listed in Table 1, showing the high photometric quality of the runs and hence of the data (notice that in sigma any actual signal will be included). [FIGURE 1] Figure 1: Amplitude spectra of the residual data series obtained on the star 18 Vul. Notice the double peak at frequencies between 0 and 0.25 mHz associated to an unresolved double mode pulsation of the star. a: For the night of October 18th, b: Id. 19th, c: Id. 20th. Table 1: Journal of the observations. Date t ... K_B (hours) (mmag) (mag/am) 18/10/88 4.2 5.8 0.23 19/10/88 4.5 2.9 0.25 20/10/88 4.5 4.8 0.23 Nightly data series were analyzed via an Iterative Sine Wave Fitting (ISWF) procedure for unevenly spaced data (Ponman, 1981; Belmonte, 1986) to obtain, for each night, the amplitude spectrum. Figure 1 shows the results of the analysis. In all the plots two significant peaks are present at the low frequency range. The corresponding frequency, amplitude and phase are listed in Table 2. Table 2: Frequency, amplitude and phase of the two main peaks present in the amplitude periodogram of Figure 1. Origin for the phase 0h UT October 18. Date nu A_1 phi_1 nu_2 A_2 phi_2 (micro Hz)(mmag) (rad) (micro Hz) (mmag) (rad) 18 90 6.3 0.3 180 3.4 2.8 19 90 2.4 0.1 200 1.3 1.9 20 90 3.8 0.3 180 3.5 2.4 In order to obtain more information, to decrease the noise and to increase the resolution, a single time series was produced using the 3 nightly series. This series was again analyzed via ISWF. The analysis, after prewhitening of the highest amplitude peak at the lowest frequency (95.3 micro Hz), yielded the following results: P v A phi (hours) (microHz) (mmag) (rad) 2.9 95.3 3.83 1.1 1.4 201.3 1.50 0.3 From this analysis (see Figure 2), it was also obvious that 18 Vul was stable in the range from 1 hour to a few minutes, at least, at a level of 0.5 mmag. [FIGURE 2] Figure 2.: Amplitude spectrum of the whole residual series of the Am star 18 Vul (18-19-20/10/88). The main conclusion is that 18 Vul is fluctuating (pulsating?) with, at least, a main period (95.3 micro Hz) and possibly its first harmonic (201.3 micro Hz). The very low amplitude of the variations could explain the non variability found by Breger in his short series of observations. From the analysis of the complete series, there are traces that more than two frequencies are in action at once in this star which, together with the seemingly amplitude modulation, has made us to include 18 Vul in a future rapid differential photometric and spectroscopic campaign, to be soon performed. Juan A. Belmonte and Teodoro Roca Cortes Instituto de Astrofisica de Canarias 38200 La Laguna, Tenerife, Spain. References: Belmonte J.A., 1986. Tesis de Licenciatura. Universidad de Barcelona. Bertolini C., Foschini R. and Piccioni A., 1974. Comm. 27 IAU Bull. Var. Stars, 948. Breger M., 1969. Ap. J. Sup. Ser. 19, 79. [BIBCODE 1969ApJS...19...79B ] Hauck B. and Mermilliod M., 1980. Astron. Astrophys. Sup. Ser. 40,1. [BIBCODE 1980A&AS...40....1H ] Manteiga M. and Martinez Roger C., 1988. Unpublished. Ponman T., 1981. M.N.R.A.S. 196, 583. [BIBCODE 1981MNRAS.196..583P ]