jose bartoli are expected to fetch upwards of $ 120,000 when the passionate missives go up for auction later .
during her marriage to muralist diego rivera , frida penned the letters to jose , a catalan artist whom she met while she was recovering from surgery in new york .
their steamy correspondence began shortly after she returned to mexico following the trip and continued throughout their secret three-year romance .
` i do n't know how to write love letters .