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According to the Air Europa webpage on special baggage, musical instruments and other non-standard luggage are subject to restrictions. If the instrument size is less than 115 cm you can take it on-board as your one piece of hand-luggage. If the size is larger than 115cm you can either put it in the hold by paying a fee, or place it on another seat that you'll have to purchase separately.
Quoting from the linked website:
Of particular interest is this part:
Note (6): Musical instruments "<115 cm" can be taken on-board as conventional cabin baggage at no extra cost providing they are carried instead of the single piece of hand luggage allowed per person.
Note (7): Musical instruments "> 115 cm" should go in hold paying a supplement of β¬ 150 or you can take it with you in cabin on a seat by paying the price of the additional space.