Could U Aqr be coming out of its prolonged, historical minimum? It has brightened by one mag in the last fewdays.
Jim Roe [ROE]
Its definitely on an upward trend. I have seen it brighter each time I observe it, now positive estimates in the 16-mag range. A lot of "scatter" in the observations though, not sure if its real behavior or the usual "inter-observer variations"?
There does seem to be a large scatter at the moment - in excess of one magnitude between CV & Vis. One might expect some scatter due to the colouring of U Aqr at this stage in it's recovery, but +1 mag does seem a bit excessive. Still this is new ground for this star, with this being the deepest fade it has experienced. Previous fades back to the early 1900's have all been in the 15 or 16 mag range, so it's interesting to see the unfiltered CCD, V & visual data being reported.
For me the declination of -16d 52' means the field lies in the 'orange dome' of Birmingham, which equates to a loss of about 3 magnitudes in my 20 inch. I should start to recover it when it's just brighter than 14. My current data is all CV with the BRT.
Any chance of getting true Johnson V magnitudes from BRT instead of CV? Unfiltered magnitudes are always questionable, whether for a target star or its sequence, and cannot be compared with, say, visual magnitudes with any degree of accuracy. This is especially true for peculiar stars like R CrB's; see the spectrum at http://www.astrosurf.com/buil/us/peculiar2/rcrb.htm for example. That may be why your BRT measures are deviating from the visual measures.
Now that U Aqr is brightening I'll change to a V filter. The whole idea of CV was to record it deep.
From QL it looks like V is closer to CV than visual anyway - which I thought was a bit strange for an RCB star.
As it appears that I am the only visual observer during the recent brightening phase, let me review some details of my observations. I have been using the 158, 164 and 169 comps. My estimates in the mid-16 range were primarily against the 164 star, which is fairly red (it appears harder to see than expected for a 16.4). As U Aqr is also similarly red, these estimates should be pretty good. (Purkinje should affect them both in similar ways). My most recent estimate of 16.0 was primarily against the 158 comp, which is solar colored, and thus this estimate is likely to be a bit low due to Purkinje making U Aqr appear fainter in comparison due to its redder color. However, I do not adjust my estimates for color of the stars, I think AAVSO does not recommend observers do that. But its likely the latest visual estimate of U Aqr is probably more like 15.9 or 15.85, if corrected for color.
I should mention the 164 comp has a high error in the B value, so the color issue may be better or worse than previously stated.
Another added complication is that the RCB stars change color as they decline and return to maximum. Most of the time, they get redder in declines but sometimes they don't change color much. It has to do with whether the whole photosphere of the star is eclipsed by the dust. Also, when the stars are very deep in declines, most of the light seen may be scattered by dust which makes it bluer.