Stella is zone 5, self-fertile. It must be helping your Kristin. What do you think about the Kristin, jerry63? Unfortunately, I am zone 4, so suggestions from further north are still needed.

A Van or Lapins might do the trick. Lapins is self fertile too which could help with production.

I think I read Blackgold didn't bloom at the same time than Kristin, but I'd still stry it!! There's also Napoleon...I want a sweet cherry tree so bad, but didn't buy any yet because everybody told me they would not make it here... But there it is, big (appox. 4 meters high) in the front yard of a house in my city, within 2 km from my house. I noticed it over the weekend and talked to the owners. They told me they had it for 15 + years. It's a Stella. It's planted in a sheltered location, away from the winter sun and the cold winds. They are a bit closer to water than I am, that might help regulate the temp...

I had a Gold cherry that cross pollinated with my Kristin. Unfortunately both varieties succumbed to winterkill and disease.

Windsor has a reputation of being one of the hardier selections of Sweet Cherry