D'Addario Prelude Viola set: 

Prelude viola strings are manufactured using a solid steel core. Unaffected by temperature and humidity changes, they have excellent bow response. Prelude strings have a warm sound in a solid steel core string design. Packaged in uniquely-designed sealed pouches providing protection from the elements that cause corrosion.

Includes:

• Prelude Viola A (J911SM) - aluminum/steel: medium gauge
• Prelude Viola D (J912SM)- aluminum/steel: medium gauge
• Prelude Viola G (J913SM)- nickel/steel: medium gauge
• Prelude Viola C (J914SM)- nickel/steel: medium gauge