Well, at least I don't know what it is. This rose has been in a hedge of forsythia which is at the back of my "street" garden behind my and the neighbor's house. It blooms for a brief period in June with small, perhaps a couple of inches wide, soft yellow single flowers. It is a lovely thing in bloom, but it seems out of place, did my dad's aunt plant it there before my parents inherited the house or is it the result of an errant seed dropped by a bird? I have kept the forsythias under firm control since establishing the garden, keeping them only because my mom really likes them, but I have favored the mystery rose bush, since I have never seen it elsewhere. It most closely resembles Rosa pimpinellifolia, though the flower color seems a bit out of the ordinary for this species. It does have the small leaves, numerous leaflets, profuse small thorns, and drab fruit of R. pimpellifolia, so maybe it is a yellow variant thereof. I rather like it, despite its short lived floral display.