Three senators are under fire amid allegations that they live in the Ottawa fulltime but collect living expenses available under Senate rules to those who must travel to and from the National Capital Region for work.

One of the three, Conservative Senator Mike Duffy, is also accused of not living in the province he was appointed to represent, a requirement under the Constitution that he own and maintain property there and be a "resident."

Senators who live more than 100 kilometres outside Ottawa can have a second residence in the National Capital Region and receive up to $21,000 a year to cover that expense.

Last week, the upper chamber announced it has asked an outside auditor to examine the residency declarations and expense claims of the three senators — and that it's seeking legal advice on the status of Duffy's residence.

Duffy says he does live at the home he owns in Cavendish, P.E.I.

However, CBC News has learned that he doesn't collect a provincial tax credit — available on properties where the owner resides in P.E.I. for at least half of the year — and that he requested a new health card, not a renewal, from the province in December.

"I represent taxpayers with care, and Canadians know I would never do anything to betray the public trust," Duffy said in a statement last week. "I have a home in Prince Edward Island as required by law."

Duffy has claimed $42,802.43 in living expenses in Ottawa since Sept. 1, 2010, the earliest date for which expenses are published.

Who else is facing scrutiny?

Senator Patrick Brazeau and Liberal Senator Mac Harb were identified in media reports last fall, along with Duffy, which alleged that they claim living expenses but don't reside at their designated primary homes outside the capital.

Their files were also sent for review by auditing firm Deloitte along with Duffy's.

Brazeau, who was kicked out of the Conservative caucus Feb. 8 after he was charged with assault and sexual assault, says his primary residence is in Maniwaki, Que., 135 kilometres from Ottawa.

However, neighbours and members of the community says that he is rarely, if ever, there.

Brazeau, who sits as a senator from Quebec and also lives in Gatineau, Que., collected $36,701.08 in living expenses since September 2010.

Harb, who was appointed to the Senate from the House of Commons, represents Ontario and reportedly lists a home in Pembroke, Ont., as his primary residence, about 150 kilometres from Ottawa.

He has collected $40,212.12 since September 2010.

What are the main issues?

According to Peter Russell, professor emeritus at the University of Toronto and one of the foremost experts on the Constitution, there are two separate issues at play.

The first concerns allegations that senators have been collecting living expenses for commuting at least 100 kilometres to Ottawa when they actually live in the capital or nearby.

The second issue is one of residency and the requirement that a senator live in the province they represent, Russell said.

"[Duffy] seems to be on its own," Russell said. "He may have an expense issue, but that is not really part of the residence issue."

Although Harb and Brazeau are under fire for expenses, no one has suggested that they do not live in the provinces they were appointed to represent.

What does the Constitution say about residency?

Senators, who are appointed by the governor general on the advice of the prime minister, represent a specific province or territory, according to a formula set out in the Constitution.

For instance, Ontario and Quebec each get 24 senators, Nova Scotia and New Brunswick get 10, P.E.I. gets four senators and the rest of the provinces get six each. The territories each have one senator.

The Constitution lays out the qualifications for senators in Section 23, including that they must "be resident in the Province for which he is appointed."

This is important, Russell says, because senators must have an understanding of local issues to properly represent their respective provinces or territories.

"It's just fundamental," Russell says.

Could Duffy lose his seat?

Section 31 of the Constitution lays out the conditions which can lead to a senator losing his or her seat, including for failing to be qualified "in respect of Property or Residence" apart from the need to stay in Ottawa for the performance of Senate duties.

Russell said failing to live in the province from which a person is appointed could be grounds for removal, because it violates the principle that senators represent a particular region.

"That's how I interpret it," he said.

On the other hand, Conservative Senator David Tkachuk, chairman of the committee on internal economy, said last week that the Senate may look at changing its own rules about residency and submitting expenses.

One of the problems, he says, is that the constitutional requirement that a senator be a "resident' of the province he or she represents doesn't define what is meant by the word.

"The Constitution is quite nebulous about this," Tkachuk said. "It says residence, it doesn't say primary residence."

Removing a senator is ultimately up to the Senate itself according to Section 33 of the Constitution, and it doesn’t appear that any seat has been declared vacant against the wishes of its occupant.