Known as Arnu, the 18 year-old is moving to Real Madrid on-loan to June with an option to buy.
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Arnu will be registered with Castilla, where their attacking options have been hit hard by injury.
The teen is a Spain U17 international and has also made his LaLiga debut with Valladolid.
Arnu becomes López de Lerma's first signing since taking charge of Castilla after Alvaro Arbeloa left to take the senior Real Madrid job.