Before my 19-year-old son di3d, he donated his heart to an 18-year-old girl — after the transplant, her mother handed me a letter from my son that began, “Mom, there’s something you need to know about her.”

Lena’s mother.

Her name is Katrin. She was twenty-eight when Lena was born.

She did not know Martin was married. That is what she says and I have decided to believe it, and I will explain why further down.

He told her he was separated. He was around for the first eleven months of Lena’s life, on and off, and then he stopped coming, and she never chased him, because she had worked out by then what he was.

Lena grew up knowing her father was called Martin and that he was not part of her life. No photographs. No contact.

And then in the July of last year, a nineteen-year-old boy sent her daughter a message saying I think we’ve got the same dad and there’s something you need to know about your health.

Katrin knew immediately, she told me, that he was the wife’s son. She had always known there was a wife. She had worked that out in 2007 from a Christmas card she was not supposed to see.