Tungsten on steel rope core. Medium gauge
Pros: I tried several different combinations of strings lent to me by my teacher before settling on Pirastro Permanent. On my instrument, the Permanent strings had a depth of sound unmatched by any except Larsen with particularly good performance in higher thumb positions. Also, they are nice and smooth which makes shifting much easier. The set of Permanent seemed better matched to one another than the Larsen set, although some people recommend Larsen A/D and Permanent G/C. My teacher recommended Permanent 'Soloist' for D/G/C and regular Permanent for A. I am delighted with the result.
Cons: Not cheap, but worth it.