@Mischa, the app Cyclers work with Strava and support heart rate using the Bluetooth Zephyr monitor
Heart rate monitor for Window Phone working with Cyclers app. Support Bluetooth heart rate monitor ZEPHYR HXM around $75.00
I am using Cyclers. It working good with my Bluetooth heart rate sensor. You can be cycling or running. Crash once in a while but working good.
Another user waiting for the Windows Phone app here. (May 21 2014)