Single Thomastik Spirocore C string - Tungsten. (Model S33). 

Spirocore strings are a technological advancement over conventional rope-core strings because they have a more flexible spiral core made of steel, creating a higher period of vibration than other rope core strings.The hi-tech core makes for effortless fingering, responsive bowing, stable tuning and a very long string life. The Spirocore G and C cello strings are available in Chrome, Silver and Tungsten.