2019A&A...625A..13M

Aims. Young double star V1318 Cyg, which is associated with a small isolated star-forming region around HAeBe star BD+40°4124, has very unusual photometric and spectral behavior. We present results of photometric and spectroscopic observations in the optical range.
Methods. We carried out BVRI CCD photometric observations of V1318 Cyg from 2015 Sept. to 2017 July. For the same period we acquired medium- and low-resolution spectra. Observations were performed with the 2.6m telescope of the Byurakan observatory. We also analyzed the images of this field in IPHAS and other surveys.
Results. We analyze the historical light curve for V1318 Cyg and demonstrate that the southern component, V1318 Cyg S, after being rather bright in the 1970s (V∼14mag) started to lower its brightness and in 1990 became practically invisible in the optical. After its reappearance in the second half of the 1990s the star started to become very slowly brighter. Between 2006 and 2010 V1318 Cyg S started brightening more quickly, and in 2015 had become brighter by more than five magnitudes in visible light. Since this time V1318 Cyg S has remained at this maximum. Its spectrum shows little variability and consists of a mixture of emission and absorption lines, which has allowed for estimates of its spectral type as early Ae, with obvious evidence of matter outflow. We derive its current A_V~=7.2 and L=750L_☉ thus confirming that V1318 Cyg S should belong to the Herbig Ae stars, making it, along with BD+40°4124 and V1686 Cyg, the third luminous young star in the group. It is very probable that we observe V1318 Cyg S near the pole and that the inclination of its dense and slow (∼100km/s) outflow is low.
Conclusions. The unusual variability and other features of V1318 Cyg S make it difficult to classify this star among known types of eruptive young stars. It could be an extreme, higher-mass example of an EXor, or an object of intermediate class between EXors and FUors, like V1647 Ori.