RCB star declines
It was great to see Mike Simonson's review of RCB lightcurves in the latest AAVSO newsletter. It is cool that so many stars, including R CrB, itself, are in deep declines. It is still a bit of a mysetry why some of these stars seem to be in decline all the time and some very rarely go down. The only good study I know of that shows which stars are the most active is Jurcsik (1996, http://adsabs.harvard.edu/abs/1996AcA....46..325J). As Mike pointed out, one of the least active RCB stars is MV Sgr. Its only known declines are reported in Hoffleit (1959, http://adsabs.harvard.edu/abs/1959AJ.....64..241H). MV Sgr is also unusual in that it is one of the rare hot (Temperature ~ 20,000 K) RCB stars. The apparent change in amplitude does look strange. If you look at Figure 8 of De Marco et al. (2002, http://adsabs.harvard.edu/abs/2002AJ....123.3387D), it looks like MV Sgr has faded about a magnitude in the last 50 years. Keep observing it!