For the nine months ended September 30, 2019, depreciation and amortization expense increased $0.3 million, or 0.3%, as compared to the nine months ended September 30, 2018.
Depreciation and amortization expense generated by the 21 Hotels increased $5.6 million in the first nine months of 2019 as compared to the same period in 2018, primarily due to the same reasons noted above in the discussion regarding the third quarter. These increases to depreciation and amortization expense were further offset by decreases in the amortization of intangible assets, consisting of advanced deposits related to our purchase of the Boston Park Plaza, which were fully amortized in June 2018.
The Six Sold Hotels caused depreciation and amortization to decrease by $5.3 million in the first nine months of 2019 as compared to the same period in 2018.
Impairment loss. Impairment loss was zero for both the three and nine months ended September 30, 2019, and zero and $1.4 million for the three and nine months ended September 30, 2018, respectively. During the second quarter of 2018, we recorded an impairment loss of $1.4 million on two hotels that we subsequently sold in October 2018.
Interest and other income. Interest and other income increased $1.2 million, or 45.1%, during the three months ended September 30, 2019 as compared to the three months ended September 30, 2018, primarily due to higher interest rates. During the third quarter of 2019, we recognized $3.6 million in interest income and $0.1 million in energy rebates due to energy efficient renovations at our hotels. During the third quarter of 2018, we recognized $2.6 million in interest and miscellaneous income.
For the nine months ended September 30, 2019, interest and other income increased $6.4 million, or 91.5%, as compared to the nine months ended September 30, 2018, primarily due to higher interest rates. During the first nine months of 2019, we recognized $11.1 million in interest income, $1.0 million related to an area of protection agreement with Hyatt Corporation for the Hyatt Regency San Francisco, $0.9 million related to a contingency funding payment received from the prior owner of one of our hotels, $0.2 million in energy rebates and $0.3 million in vendor rebates and other miscellaneous income. During the first nine months of 2018, we recognized $5.8 million in interest and miscellaneous income, along with $1.1 million in insurance proceeds for hurricane-related property damage at two hotels we subsequently sold in October 2018 and $0.1 million in energy rebates.
Interest expense. We incurred interest expense as follows (in thousands):
| | | | | | | | | | | | | |
| | Three Months Ended September 30, | | Nine Months Ended September 30, | |
| | 2019 | | 2018 | | 2019 | | 2018 | |
Interest expense on debt and finance lease obligations | | $ | 11,406 | | $ | 11,619 | | $ | 34,399 | | $ | 34,364 | |
Noncash interest on derivatives and finance lease obligations, net | | | 1,155 | | | (818) | | | 6,908 | | | (4,995) | |
Amortization of deferred financing costs | | | 698 | | | 748 | | | 2,094 | | | 2,240 | |
Total interest expense | | $ | 13,259 | | $ | 11,549 | | $ | 43,401 | | $ | 31,609 | |
Interest expense increased $1.7 million, or 14.8%, during the three months ended September 30, 2019 as compared to the three months ended September 30, 2018, and increased $11.8 million, or 37.3%, during the nine months ended September 30, 2019 as compared to the nine months ended September 30, 2018.
The increase in interest expense for both the three and nine months ended September 30, 2019 as compared to the same periods in 2018 is primarily due to the noncash changes in the fair market values of our derivatives, which caused interest expense to increase $2.0 million and $11.9 million, respectively.
Excluding the noncash impact from changes in the fair market values of our derivatives, interest expense would have decreased $0.3 million and $0.1 million in the third quarter and first nine months of 2019, respectively, as compared to the same periods in 2018, due to lower debt balances and lower deferred financing costs resulting from monthly amortization, and lower interest expense on our term loans, which we amended and repriced in October 2018. These decreases were partially offset by higher interest on our variable rate debt during both the third quarter and first nine months of 2019 as compared to the same periods in 2018.
Our weighted average interest rate per annum, including our variable rate debt obligation, was approximately 4.2% at both September 30, 2019 and 2018. Approximately 77.5% and 77.7% of our outstanding notes payable had fixed interest rates at September 30, 2019 and 2018, respectively.
Gain on sale of assets. Gain on sale of assets totaled zero for both the three and nine months ended September 30, 2019, and $53.1 million and $68.8 million for the three and nine months ended September 30, 2018, respectively. During the first nine months of