Activist investor ValueAct Capital Partners LP has built a roughly $1.2 billion stake in Citigroup Inc. over the past four to five months, The Wall Street Journal reported Monday, citing a letter it reviewed. Shares rose more than 1% late Monday. The bet is that the bank's strength as a service provider to corporations will enable it to thrive and make up for the ground its shares have lost in recent years, according to the report. In the letter, ValueAct says it has continued to boost its stake "opportunistically," according to the report. A spokeswoman for Citigroup told the newspaper that the bank is having "constructive conversations with ValueAct" and it welcomes them as investors. Citigroup shares ended the regular session up 0.8%.