Is there a brown dwarf orbiting our sun?

Answer

That story is not new, this hypothesis was put forward several decades ago. But in any case it is still a pure hypothesis, so not a theory at all. And in any case, no such brown dwarf has been observed.
The brown dwarf is believed to be responsible for a periodic perturbation of the Oort cloud that would increase the likelihood of comet impacts on Earth every approximately 26 million years.
First, whether that periodicity exists is not yet certain. A number of possible mass extinction events have been observed with an average interval of 26 million years. Second, the Oort cloud, the supposed reservoir of comets, is still a hypothesis for which no observational evidence has yet been found, although astronomers consider its presence very likely. And as said, no trace of that brown dwarf has yet been observed. So what have we already observed concretely…?
So, everything remains an assumption. Another explanation, IF that periodicity is there, and IF the Oort Cloud exists, would not be a brown dwarf, but the global motion of the sun and everything around it, in the galaxy. The sun passes through the plane of the galaxy every few tens of millions of years, so COULD cause a disruption of the Oort cloud.
So, the story is a hypothesis, a possibility, for which there is no evidence yet.
see also : http://www.absoluteastronomy.com/topics/Nemesis_%28star%29