This is what D’Addario have to say about these strings:
XS Nickel coated electric guitar strings provide the longest life of any D’Addario electric guitar string. Coated with our groundbreaking XS coating, these premium electric strings attain maximum life and a smooth, fast feel without dampening their outstanding tone. Constructed with a NY Steel core, reformulated nickel-plated steel wrap wire, and Fusion Twist technology, XS Nickel strings boast superior strength and tuning stability, plus richer harmonics, greater clarity, and remarkable definition with every sweep, slide, and bend. This 9-42 Super Light gauge set is easy to bend and very flexible.