I have discovered that Gen Auguste Mercier (1833-1921) married Fanny Isobel Tremayne Simons at Versailles in 1871.
They had two sons, both of whom joined the Army.
One was Evelyn Gabriel Tremayne Mercier, born 1876, who was promoted to Lieut-Colonel in the infantry.
The other was Francis Auguste Tremayne Mercier, who was a Dragoons officer.
How can I find out more about these men?