W Serpentis

W Serpentis is a multiple star in the constellation Serpens. The system varies between apparent magnitudes 8.42 and 10.2 over a period of just over 14 days, too faint to be seen with the unaided eye. This is mainly due to the system being an eclipsing binary (that is, one star passing in front of the other star and resulting in a change in brightness). However, variations in its period indicate there are some innate changes in luminosity of one or both component stars as they interact with each other, and it has been difficult to disentangle the light to determine their nature. The period is increasing by 14 seconds a year, indicating that a massive amount of material is being transferred from the larger fainter star to the smaller brighter one.
The system has been found to contain an accretion disk, and was one of the first discovered Serpentids, which are eclipsing binaries containing exceptionally strong far-ultraviolet spectral lines. These systems have a high rate of mass transfer between one star and the other, and are thought to evolve first into double periodic variables and then classical Algol variables. In the late stages of mass transfer, such systems can develop an optically thick disc and are segregated as a separate class of W Serpentis stars. The classes may also represent different types of system altogether, with the W Serpentis type being younger and more massive.
The spectral types of the component stars are not known; the hotter brighter star has been classed as an F-type giant. However, it appears to be completely obscured by material, and another theory holds that it is a hot main sequence star with the surrounding material responsible for the spectrum as it appears to be too luminous to be an F-class star. The spectrum of the secondary (larger and fainter) star has not been detected either. Examining the polarised light shows that a jet of material is probably coming from the primary star as well.
Further reading.
https://www.eso.org/sci/publications/messenger/archive/no.39-mar85/messenger-no39-40-42.pdf