This is the commonest (hence the name) of a group of Erebia known collectively as "brassy" ringlets, as they all exhibit a brassy sheen on the upperside, often visible in flight. It may be the commonest of the brassy ringlets, but I would not describe it as common, per se. 

The Col de Vars in the Hautes-Alpes is at an of altitude 2100m. Until 2006 this was the only place I had seen cassioides and when I first saw it in 2004 it took a few moments to realise what it was, especially as the underside has a silvery appearance in flight. Since then I have found it regularly at high altitude locations, always above 1800m. Lafranchis gives the minimum altitude as 1400m.

This species is referred to as Western Brassy Ringlet (Erebia arvernensis) in Lafranchis' ID book, and shown as occurring principally in the western Alpes and the Pyrénées, whereas cassioides is shown as occurring in the eastern Alpes and localities in the Balkans. This is somewhat confusing as most textbooks (including Lafranchis' earlier work on French butterflies TLFr) show cassioides as occurring in the Alpes and Pyrénées, with no reference to arvernensis which had not then been designated as a separate species.

T&L refers to arvernensis as being the subspecies (of cassioides) that occurs in the Pyrénées, the Massif Central, and peninsular Italy. In the 2017 reclassification, arvernensis was split off from cassioides, and the new taxonomy states that it is arvernensis that flies in France (despite what T&L previously said), so all on this page are now considered as arvernensis.

observations

a male, with the brassy sheen shown clearly by the green reflections. Fairly typical cassioides with the twin ocelli slightly offset from each other and having the red post-discal area just surrounding the ocelli.

a male, I think, even though the upf red post-discal band extends strongly to s4, less so to s3 and vestigial in s2, where the longer band would tend to indicate female. I am basing the assumption that it is a male, taking salts from dry-ish ground (females take moisture, but only from wet or damp ground?), and that the just-visible underside is more greyish and appears to lack the brownish colouring of the female underside. The fringes are also almost uniformly dark, whereas I would have expected them to be rather lighter or more chequered for a female. Either way, the shot catches the green reflections nicely, illustrating why they are called brassy ringlets.

a very fresh male, as indicated by the straightness of the forewing margins and the body shape. The red upf post-discal band extends down to s3, lower than would be expected for a male.

a female, as clearly indicated by the body shape. It was in the right territory for the Pyrenean Brassy Ringlet (E. rondoui), but rondoui has the upf ocelli merged to the extent that it is a single ocellus with two white centres.

a male, as indicated by the clean silvery-grey colouring.