The Duke of Arco is a Spanish noble title created by King Philip V in 1715 for Alonso Manrique de Lara y Silva.
Succession
As with other Spanish noble titles, the dukedom of Arco descended according to cognatic primogeniture, meaning that females could inherit the title if they had no brothers (or if their brothers had no issue), such as in the 7th Duchess's case. The rules were relaxed in 2006, since when the eldest child (regardless of gender) can automatically succeed to noble family titles.
The 7th Duchess married the late Fernando de Soto y Colón de Carvajal (1930-2001), V Count of Puerto Hermoso, X Marquis of Arienzo, and it is expected that their son, Manuel de Soto y Falcó, will succeed to his mother's dukedom.
References
Duke of Arco Wikipedia(Text) CC BY-SA