This weeks light curve of the week features the emission line star EM* AS 405. It is situated in the area of the target field Berkeley 86 in Cygnus. The Gaia parallax places the star at a distance of about 3kpc or 9000 light years. Hence, much further in the background than the target cluster in the centre of the field. The spectral type of the star is B1, so we have a slightly massive star, situated in the background.

The light curve data shows that the star is significantly brighter in Hydrogen alpha than in R, confirming it’s nature as an emission line object. Generally there is no noticeable long term trend in the objects brightness, but the detectable fluctuations occur on short days to weeks timescales. The overall brightness fluctuations do not exceed 0.2mag in all filters, except for some outlier data points.

This object is one of the examples of variable stars detected in our fields which are not associated with our target clusters and star forming regions. By their nature the detectable background variable stars are all rather more massive. These massive stars are rare in our nearby target regions and we are hence able to investigate them further in the background instead. With Gaia parallaxes now available for almost all stars in our fields, we can now investigate their intrinsic properties much better than before Gaia.