While monitoring an Arhopala female flying around its host plant, a Baron-typed butt flew within sight and perched itself on a nearby tree-top. I took three cheap shots (see pic 1 for one), thinking that it is a Horsefield's Baron and forgot about it for the rest of the outing.
The review session on my home PC however revealed a butt which I have not seen before (see pic 2 and 3 for cropped portions showing the wing markings). Checking both C&P4 and Pisuth's book shows that it closely matches a female Eulaceura osteria kumana (The Elegant Emperor).