SRA V1118 HER

Hi Peter - I looked up V1118 Her in the AAVSO and ASAS databases and, indeed, it displays alternating maxima and minima in its lightcurve much like an RV Tauri (RVT) variable. RVTs are somewhat rare and this is probably because they are undergoing a rather rapid phase in their evolution. Because of this rapid evolution we just can't capture many at a given time. RVTs are generally yellow or orangish giants (G and K spectral types) and are thought to be low mass objects (around one solar mass at the beginning of their lives and about 0.6 solar masses when they are showing RVT characteristics). V1118 Her, however, does look to be a very red RVT-type  but also as an M class SRa variable. On rare occasions I have come across other similar variables. I don't think there is yet a consensus of opinion as to what causes the alternating amplitudes. I recall that in the 1990s a number of big name stellar pulsation researchers were putting a lot of study into this, but found no definitive cause. They looked at such possibilities as chaotic dynamics (chaos) theories causing intersecting shock waves in the outer stellar layers finding a resonance of sorts, etc. I haven't kept up with recent work but I'm thinking this is still an open question and there is still more work to be done. I encourage you to keep observing V1118 Her and for you and more more observers to look at RVTs overall of all colors. And why a late M-type variable is behaving like a yellow RVT adds to the thrill of the chase. There is some really interesting stellar evolution going on here and collecting more and more data on these stars will likely help solve the puzzle someday!