Common Blue habitually congregate at dusk just before they go to roost. I saw eight males in a fairly small square area of about about 2' but these two were the only ones together. It was much duller yesterday evening and what a difference the weather makes. Common Blue was the only butterfly species I saw last night at this site.
Pair of Common Blue preparing to go to roost.
Taken with D3x Nikon and 105mm Macro. Tripod mounted. More images can be found on my Flickr site here