NEW YORK (TheStreet) -- Tesla Motors' (TSLA) efforts to merge with SolarCity (SCTY) got a big boost Friday when shareholder advisory firm Institutional Shareholder Services recommended that the companies' investors approve the deal.

Elon Musk's effort to unite the two companies he holds deep ties to has been on the table since June 21, the bid was originally valued at $2.8 billion. Musk is CEO of the automaker and chairman of the solar energy company.

There are some that are still skeptical of the deal despite ISS' vote of confidence.

"I am very cautious on the deal," UBS analyst Colin Langan said on CNBC's "Power Lunch" Friday. "If you look at SolarCity it's, from an operating perspective, burning through cash. Tesla itself is burning through cash. That's adding more risk on the company."

Shares of Tesla and SolarCity were higher in Friday afternoon trading, with SolarCity shares jumping up 9.35% to $20.28.

Separately, TheStreet Ratings objectively rated this stock according to its "risk-adjusted" total return prospect over a 12-month investment horizon. Not based on the news in any given day, the rating may differ from Jim Cramer's view or that of this articles's author. TheStreet Ratings has this to say about the recommendation:

We rate TESLA MOTORS INC as a Sell with a ratings score of D+. This is driven by a number of negative factors, which we believe should have a greater impact than any strengths, and could make it more difficult for investors to achieve positive results compared to most of the stocks we cover. Among the areas we feel are negative, one of the most important has been a generally disappointing historical performance in the stock itself.

You can view the full analysis from the report here: TSLA

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