That serif is called a cupped serif. How the curve itself is called I don't know.

In german, this is called "Kehlung", which my dictionary translate as "hollow moulding (architecture)" (which might not be a good translation).

Hans Rudolf Bosshard calls it »konkave Basis« may be »concave base«.
In the same book »Technische Grundlagen zur Satzherstellung«, Kehlung ist the Position above from serife to stem.

:) Danke. Man lernt nie aus...