The large female platy (the only female in the first bunch) gave birth to babies a day or two before she died. Since the tank was moderately planted, a few survived. I noticed that they were able to dive down between the gravel, which probably provided a better hiding place than the plants. To begin with I saw two or three babies. After a few days they became bolder, since the adults didn't both them. Eventually I figured there were at least four fry, and eventually confirmed a fifth.
For a few days I accepted the figure of five, but yesterday I started to wonder if there weren't more. This morning I confirmed 10 fry, some happily nibbling at flakes on the surface, while others were pecking at the Cory food on the bottom.