The SMC B-type supergiant AzV322: a g-mode pulsator with a circumstellar disc

We present a photometric and spectroscopic study of AzV322, an emission line object located in the Small Magellanic Cloud previously classified between O9 and B0. We analyse 17.5 yr of I-and V-band OGLE-II, -III and -IV light curves and find four significant frequencies, viz. f(1) = 0.386 549 +/- 0.000 003, f(2) = 0.101 177 +/- 0.000 005, f(3) = 0.487 726 +/- 0.000 015 and f(4) = 0.874 302 +/- 0.000 020 cycles d(-1). The f(1) frequency (period 2.587 00 +/- 0.000 02 d) provides the stronger periodogram peak and gives a single wave light curve of full amplitude 0.066 mag in the I band. High-resolution optical spectroscopy confirms the early B-type spectral type and reveals prominent double peak Balmer, Paschen, OI 8446 and He I 5875 emissions. The spectral energy distribution shows significant colour excess towards long wavelengths possibly attributed to free-free emission in a disc -like envelope. Our analysis yields T-eff = 23 000 +/- 1500 K, log g = 3.0 +/- 0.5, M = 16 +/- 1 M-circle dot, R = 31.0 +/- 1.1 R-circle dot and L-bol = 10(4.87 +/- 0.06) L-circle dot. AzV322 might be a member of the new class of slowly pulsating B supergiants introduced by Saio et al. and documented by Lefever, Puls & Aerts; however its circumstellar disc makes it an hither to unique object. Furthermore, we notice that an O-C analysis for f(1) reveals quasi-cyclic changes for the times of maximum in a time-scale of 20 yr, which might indicate a light-travel time effect in a very wide orbit binary with an undetected stellar component.